Fatestealer was a dagger drafted by Smith Dandi in Nedaria's Landing from specifications provided by an unknown adventurer (the player). The specifications were gathered from thief guilds around the world, essentially yielding an amalgamation of the best available technology of the day into a single design. According to Vorla Diensk, it was "a blade of assassination, potentially capable of slaying even the most powerful creatures."
The weapon was known as the end result of the Rogue Epic 1.5 quest series in the original EverQuest.
There is no information whatsoever that the epic Fatestealer dagger survived The Shattering, and it is therefore not part of Everquest II canon lore.