Sigil of Dark Simulacrum

Sigil of Dark Simulacrum is the death knight class tome found in Ashran.

Source
This item has a chance to drop from any of the rare elite mobs found in the secondary areas of Ashran.

Trivia
The previous version of this tome was Sigil of Necromancy - "Slain players and creatures summon a Skeletal Warrior or Mage at the command of the Death Knight. The Death Knight can have up to 3 Summons at one time."