Arthas Menethil, Crown Prince of Lordaeron and Knight of the Silver Hand, was the son of King Terenas Menethil II and heir to the throne. He was trained as a paladin by Uther the Lightbringer, and had a romantic friendship with the sorceress Jaina Proudmoore. But despite his promising beginning, Arthas became one of the most powerful and evil beings Azeroth would ever experience when he merged with the Lich King. He is voiced by Justin Gross.
Troubles began to stir in Lordaeron. Orcs broke free of their internment camps, and there was distressing news of a plague that had gripped the northlands. Arthas and Uther were sent to Strahnbrad to defend the town from orcish raids. The young prince defeated the black drake Searinox to retrieve its heart for the dwarf Feranor Steeltoe to forge into an orb of fire. Arthas used this magical item to kill the orc blademaster leading the raids.
However, a more vile threat arose in the form of the plague of undeath. Jaina and the Captain were sent to join Arthas, and they were to investigate the strange plague. They fought an undead army and a plague-infested granary. There, they encountered the necromancer Kel'Thuzad in the town of Brill, and they pursued him to Andorhal.
Kel'Thuzad had already infected the stored grain in Andorhal and shipped it out to outlying villages. Before Arthas killed him, Kel'Thuzad spoke of Mal'Ganis, a Nathrezim demon who led the Scourge. Jaina and Arthas travelled north to confront him in Stratholme.
Along the way, Arthas and Jaina stopped in Hearthglen, where they hoped to rest. Instead, they were warned of an approaching army of undead. Arthas ordered Jaina to find Uther and seek his aid, while he remained behind to defend the town. Then, to his horror, Arthas discovered that the plague was not merely a means of mass murder, but rather, the means of turning innocent townsfolk into undead creatures. Arthas' forces barely held out, and were on the verge of defeat when Uther arrived with reinforcements and saved the village.
While travelling to Stratholme, Arthas was met by the enigmatic prophet Medivh. He gave him the same advice he gave Terenas--to travel west to Kalimdor. Arthas argued that his place was with his people, and vowed that he would not abandon them. Jaina suggested that the prophet might be right, but Arthas paid her no heed, and continued to Stratholme.
When he arrived, Arthas found that the grain had already been distributed, and knew that the villagers would soon become undead. He ordered Uther and his knights to purge the entire town. Horrified, Uther rebuked him, saying that he would not follow such an order even if Arthas were already king. Proclaiming that Uther had committed treason, Arthas disbanded the Knights of the Silver Hand. Several of his soldiers left with Uther--as did Jaina--while those that remained loyal to Arthas joined him in slaughtering the corrupted townsfolk.
As Arthas began to slay the citizens of Stratholme, he was met by Mal'Ganis himself, who was working to claim the souls of the townsfolk. Arthas worked to destroy them before Mal'Ganis could reach them. Finally, Arthas demanded a final showdown with the dreadlord. Mal'Ganis slipped away, however, vowing to meet him in Northrend.
Arthas followed him with a detachment of his troops. They arrived a month later in Daggercap Bay. As they searched for a proper place to set up camp, Arthas came upon his good friend, Muradin Bronzebeard. At first, Muradin thought that Arthas was leading a rescue party sent to save his men, who had been besieged by the undead as they searched for the runeblade Frostmourne. Muradin and Arthas destroyed the nearby undead camp, but there was still no sign of Mal'Ganis.
As Muradin and Arthas went to search for Frostmourne, an emissary from Lordaeron arrived in a zeppelin and spoke to the Captain. He bore orders from Uther and Terenas, instructing Arthas and his men to return home. When Arthas returned to his base, the men had abandoned their posts, and were making their way through the forest towards their boats. Arthas had no intention of leaving before Mal'Ganis was destroyed. With the help of some indigenous mercenaries, he managed to reach and burn his boats before his men got to them. When his men arrived, Arthas betrayed his mercenaries, accusing them of destroying the ships, and the Captain had them killed. Arthas told his men that they had no means of going home, and the only way they were leaving Northrend was through victory.
Arthas and his troops continued to press on to Draktharon Keep in search of Frostmourne. As he arrived, Mal'Ganis appeared to him, and foretold his death. Arthas went to search for Frostmourne with Muradin, leaving his captain to defend the camp.
Arthas soon found the Guardian, who tried to keep him from Frostmourne, apparently for his own protection. The Guardian fell, and Arthas and Muradin claimed their prize. But Muradin, reading an inscription, reported that the blade was cursed. Arthas asked the spirits of the cavern for the sword to be released from its icy prison, proclaiming that he would "pay any price" to save his people. When the weapon broke free, Muradin was killed by a rogue shard of ice, with no remorse from Arthas. With Frostmourne in hand, Arthas defeated Mal'Ganis' minions, and finally confronted the demon. Mal'Ganis told him that the voice he was now hearing was that of the Lich King. However, Arthas replied that voice was instructing him to destroy Mal'Ganis. Slaying the dreadlord, Arthas fled into the frozen north, leaving his troops to fend for themselves. Arthas soon lost the last remnants of sanity
The Death Knight:
Arthas travelled back to Lordaeron months later, and the kingdom rejoiced at the return of its champion. Arthas knelt before the throne of his father, King Terenas. However, Arthas rose and drew Frostmourne; and killed his stunned father with the sword.
Arthas fled the scene and was not heard from for weeks, then reappeared in Vandermar village at the bidding of his new master, the Lich King. There, he met Tichondrius the Darkener, a dreadlord, like Mal'Ganis. Thinking that the Dreadlord was Mal'Ganis out for revenge, Arthas immediately threatened him, only to discover that this Dreadlord had come to congratulate Arthas in his efforts. When spoken to Arthas said he no longer felt remorse for any of his actions. Tichondrius explained that the sword, which he claimed was forged by the Lich King, was designed to steal souls, and that Arthas' was simply the first one it claimed.
Arthas assembled the members of the vile Cult of the Damned that were hiding in Vandermar, and was aided by their magical abilities as he traveled to Andorhal where he was to recover Kel'Thuzad's corpse. Arthas killed the paladin guarding the crypt, Gavinrad the Dire, and recovered the remains of the necromancer. This brought Kel'Thuzad's ghost into being, and he secretly instructed Arthas not to trust the dreadlords. Arthas quietly contemplated this.
Kel'Thuzad's remains were badly decomposed and needed to be taken to the mystical Sunwell in Quel'Thalas to be revived. Tichondrius sent Arthas to recover a mystical urn, which could be used to transport Kel'Thuzad's remains. However, the urn was in the keeping of the Knights of the Silver Hand. Arthas killed two paladins, Ballador the Bright and Sage Truthbearer, who both condemned Arthas' betrayal. Then he met up with Uther the Lightbringer again, who - aghast - explained to Arthas that the urn held his very father's ashes, those of King Terenas. Arthas murdered his life-long mentor and seized the urn. Abandoning his father's remains, he replaced them with those of Kel'Thuzad, then began the long journey to Quel'Thalas.
Arthas met heavy resistance from the elves, rallied by Ranger-General Sylvanas Windrunner. Driving their forces before his undead army, he steadily pushed her people back in a swath of destruction towards Silvermoon. Sylvanas tried to warn the vast elven capital of the coming of the Scourge, but Arthas destroyed her camps and killed the Ranger-General. To make her pay for her perceived insolence towards Arthas, he corrupted her elven spirit, transforming it into a horrendous twisted form (a Banshee) and enslaving her to the will of the Lich King, forcing her to slay her own people.
Arthas annihilated Silvermoon and used the Sunwell to bring Kel'Thuzad back to life reborn as an undead Lich. As the pair traveled to Alterac, Kel'Thuzad explained the full extent of the "Second Invasion" and the plans of the Lich King and the Scourge. Kel'Thuzad was in Alterac to destroy a camp of Blackrock Clan orcs who had taken control of a demon gate, which the lich would use to speak to the demon lord Archimonde. The Scourge destroyed the orcs, and after Kel'Thuzad received orders from Archimonde, they set out for the powerful seat of the world's mages, the city of Dalaran. Archimonde instructed them to acquire the spellbook of Medivh, which would allow Kel'Thuzad to summon Archimonde into Azeroth.
Despite the Kirin Tor's valiant efforts to repel the invasion, the Scourge fought through their magical defenses and fortifications, killed the Archmage Antonidas and claimed the Book of Medivh.
Arthas and his troops repelled the mages' vast counterattack as Kel'Thuzad began the lengthy summoning of the demon lord. Once Archimonde arrived, he proclaimed that the Lich King was of no further use to the Legion, and Tichondrius was placed in command of the Scourge. Arthas was left to wonder what would become of him and Kel'Thuzad, but the lich replied that all was going as the Lich King foresaw. The pair disappeared as Archimonde took his revenge on the city, destroying Dalaran with a single powerful ritual.
Arthas was next seen several months later in Kalimdor, where Tichondrius was employing the arcane power of the Skull of Gul'dan. Arthas told the recently freed demon hunter Illidan how to claim the skull's powers as his own, whereupon he could destroy Tichondrius. Illidan agreed to his plan, and Arthas disappeared again.
The Lich King of the Frozen Throne
Archimonde left a trio of dreadlords behind in the ruined palace gardens of Lordaeron to ensure that the nation remained under control, and to watch over the cunning servitors of Ner'zhul. When the demon lord was defeated, however, they were not originally aware of it. This changed several months later, when Arthas returned to reclaim his throne. He threatened the dreadlords, who immediately fled, and then called Sylvanas and Kel'Thuzad to his side. Together they crushed the remaining human refugees in the area, who were led by the paladins Dagren the Orcslayer, Halahk the Lifebringer and Magroth the Defender. However, during the battle, Arthas had a painful seizure, and felt the Lich King calling out to him. Despite his diminished powers, Arthas fought on until all the remaining humans were killed.
Little did Arthas know that the Lich King's power had dwindled to the point that Sylvanas was no longer under his control. In secret, she attended a meeting with the three dreadlords, who told her that the Lich King's power was waning, and so the time had come to claim her vengeance.
Arthas was ambushed in the capital city, and was forced to collect what loyalists he could find and fight his way through the dreadlord's forces, which included the powerful abomination Bloodfeast. As he arrived on the city's limits, he was saved by a cadre of banshees, who told him that Sylvanas had sent them to see him safely away. However, as they neared an empty spot in the forest, Arthas was attacked by Sylvanas, who shot him with a paralyzing arrow. Kel'Thuzad stepped in and chased her off at the last moment. But the Lich King's mental cries pierced Arthas' mind - he was told to return to Northrend, for forces were working to destroy the Frozen Throne.
Immediately, Arthas prepared his fleet and set sail for Northrend, leaving Kel'Thuzad behind to watch over Lordaeron.
Three weeks later, Arthas landed upon the familiar coast of Northrend, and unexpectedly found himself being attacked by blood elves led by Kael'thas, hungry for vengeance at their kingdom's destruction. Arthas was saved by Anub'arak, the former king of Azjol-Nerub. Kael warned that though the preliminary scouting force may have fallen, their main army would not be so easily defeated.
Arthas worried that he may be right, and that they would never reach Icecrown before Illidan, but Anub'arak thought differently. He suggested that they delve into the shattered kingdom of Azjol-Nerub. Seeing little other choice, Arthas agreed.
Anub'arak suggested raiding the hoard of Sapphiron, an ancient blue dragon and servant of Malygos, and equipping themselves with the dragon's treasures. Not only did they slay the dragon, but Arthas used what power he had left to raise Sapphiron into a powerful frost wyrm.
When he came to the doors of Azjol-Nerub, Arthas found himself being fired upon by dwarves, who revealed themselves to be followers of Muradin who had remained there when their leader died. Now, they were led by Muradin's underling, Baelgun. Leaving Sapphiron outside, Arthas battled through not only Baelgun's dwarves, but also the nerubian survivors, as he delved into the broken Spider Kingdom. Anub'arak's aid was invaluable, as he circumvented many traps which would have cut Arthas' stay fatally short.
When Arthas faced off against Baelgun, the dwarf warned that the shifting earth had released an ancient evil from below the kingdom. As Arthas and Anub'arak delved deeper into the kingdom, that evil became apparent - the Faceless Ones, a powerful, vicious race thought only to exist in legend. Arthas and Anub'arak even managed to defeat the incredibly powerful Forgotten One.
As they made their way to the Upper Kingdom, an earthquake collapsed part of the passage they were in, separating Anub'arak from Arthas. The young king had to rely upon his wits to see himself through several daunting booby traps before Anub'arak dug his way to Arthas. As they climbed out of Azjol-Nerub, the Lich King contacted Arthas once again, and explained that he was losing his power, because the Frozen Throne had been cracked, and energy was seeping from it. Ner'zhul replenished Arthas' powers, knowing he would need them in the coming battle.
When they finally reached the surface, they immediately had to contend with Illidan's forces. Vashj's naga and Kael's blood elves were there to contest Arthas' minions at every turn. Arthas, with Anub'arak's help, battled his way through their forces and magically activated the four Icecrown Obelisks around the glacier, opening the doors to the Frozen Throne. However, Illidan was waiting for him.
After a short but intense battle, Illidan inadvertently left himself open and Arthas took advantage of it, slicing open the demon hunter's chest with Frostmourne. Illidan collapsed in the snow, as Arthas turned towards the doors of Icecrown.
Arthas entered the hollow glacier and beheld a winding pinnacle chained to the ice. As he strode up the stairs towards his destiny, the voices of those he had forsaken flooded through his mind. He heard Muradin Bronzebeard, Uther, and Jaina calling out to him, yet he ignored them, continuing his ascent. Finally, he reached the pinnacle and before him he saw an icy cask. Inside was a suit of armour, arranged as if seated on a massive throne. Now only one voice spoke to him - the rasping whisper of Ner’zhul:
Return the blade...complete the circle...release me from this prison!
With a great cry of strength, Arthas brought Frostmourne to bear against the Lich King's icy prison and with a haunting scream, the Frozen Throne exploded, and shards of the crystal scattered on the ground. With Ner’zhul’s thorny helm at his feet, Arthas leaned forward, picked it up, and placed the unimaginably powerful artifact on his head.
"Now," Ner'zhul's voice echoed within his mind, "we are one!"
In that moment, Ner'zhul and Arthas' spirits fused into a single mighty being, just as the Lich King had always planned. Arthas as a single being ceased to exist, now one half of one of the most powerful entities Azeroth had ever known.
With that, the outer wall of Icecrown fell away, and only the single pinnacle remained. The Lich King sat silently upon the shattered throne, stared out across his new kingdom, still as a statue, and waited...
TL;DR: Arthas got driven mad by his people dying and became evil. Then he did some other stuff in Wrath of the Lich King.