The title the Kingslayer is awarded to players upon completing the achievement 
The Frozen Throne (10 player) or The Frozen Throne (25 player), which is awarded after the death of the Lich King in Icecrown Citadel.
This title may be a nod to the A Song of Ice and Fire series of novels by George R.R. Martin; the character Jaime Lannister also bears this title for assassinating an insane King.
