Thassarian was a soldier of Lordaeron during the Third War. He traveled with to Northrend to hunt . When Arthas and went missing, he went to search for them alone. He discovered Frostmourne Cavern, where he found Falric. Falric, already converted to a Death Knight by Arthas, slew Thassarian, and thus began his unlife as a death knight.
Thassarian returned to Lordaeron with Arthas and assisted in the massacre of its citizens. Arthas forced him to slay his mother, , to prove his loyalty. This horrific event continued to haunt him, and it remained perhaps his last scrap of humanity for some time.
Arthas' assault on Quel'thalas introduced Thassarian to the high elves and his brother, , at An'owyn, where he pursued the mooncrystal to open the second moongate and proceed with their attack. Thassarian nearly killed Faltora, but, upon remembering his mother, he decided to show a moment of mercy. He returned to Arthas with the mooncrystal that he recovered. Koltira later confronted Thassarian and questioned why he didn't murder his brother, hoping to convince him to regain his humanity. This attempt failed, and Thassarian killed Koltira, who was returned to unlife as another death knight. Koltira and Thassarian developed a significant friendship that lasts to this day.
Years later, the ordered his death knights under the leadership of to attack Light's Hope Chapel using the necropolis Acherus with the goal of raising the thousand or so Alliance soldiers buried beneath it to bolster the ranks of the Scourge. The attack on Light's Hope Chapel ended with a defeat for the Death Knights. Many of the family of the Death Knights were buried at Light's Hope Chapel, and the spirits of their relatives rose to convince them to lay down their weapons. Thassarian was convinced by his father, , to stand down. Mograine declared the independence of the Knights of the Ebon Blade. Thassarian confronted to accept the Knights of Ebon Blade as allies and to readmit its members as members of the Alliance.
World of WarcraftThis section concerns content exclusive to Wrath of the Lich King.
He is first encountered in Acherus: The Ebon Hold as a level 55 standing next to and . In phase two after completing  , he relocates to the Scarlet Tavern with Bloodbane, where he is involved in the following quests:
He shows up at the Battle for Light's Hope Chapel as a level 60 elite taking part in  .
Thassarian can later be found aboard the Alliance airship the Skybreaker above the skies of Icecrown. He is involved in quests that unfold The Shadow Vault among others. He also plays a part in the unfolding storyline of Wrath of The Lich King.
CataclysmThis section concerns content exclusive to Cataclysm.
The Alliance and Forsaken are fighting over control of Andorhal and the remaining Scourge presence there. The Alliance, led by Thassarian, have taken control of the western portion of the town while the Forsaken, led by Koltira Deathweaver, have taken control of the eastern portion. Over the course of the Battle for Andorhal Thassarian sends Alliance heroes to deal with the remaining scourge forces and after defeating the leaders, Thassarian and Koltira enter a truce. Some time afterwards Alliance forces launched a failed assault on the Forsaken. During the Forsaken counter attack Thassarian flees and his minion Lurid is killed. 
Even while he was in service of the Lich King, Thassarian seemed to be a noble person. After sending the player to rescue Koltira Deathweaver — against the code of the Scourge death knight — asked why he cared. Thassarian replied that, though they may have been hated enemies in life, "In death, we are brothers."
- "All I ever wished was to serve once again. For humanity, against a .
- Instead, I find that our numbers have been infiltrated by cultist scum.... and all of the signs point to Naxxanar as the origin of this rot."
- ^ a b c d e f Death Knight (manga)
- ^ Cataclysm Lore Details - World of Raids Forums
- ^ http://www.youtube.com/watch?v=Fsw78wHlhrE&feature=channel
- ^ http://forums.worldofwarcraft.com/thread.html?topicId=25968750004&postId=262602230514&sid=2000#39
- ^ http://www.blizzplanet.com/blog/comments/warcraft_death_knight_front_cover
|Ebon Hold||Scarlet Tavern||Light's Hope||Wailing Ziggurat||The Skybreaker||Andorhal|