Let it end in hellfire!
Henzell says...
  1. A temporary ally gained through killing a living monster with a stab while in {vampire form}. The thrall becomes undead, can also stab sleeping foes (see [2]), gets vampiric attacks, and learns Vampiric Draining if it knows any spells. Capped at one active at a time, with its duration scaling with Shapeshifting skill.
  2. Vampiric thrall stabbing damage takes their post-weapon melee damage, adds a flat bonus of x1.5 HD, then multiplies the total by the usual x2.5 sleep multiplier. A merfolk impaler thrall with a +0 trident does 120 damage sleep stabs, and a frost giant thrall with a +0 battleaxe does 182 damage sleep stabs.

Information based on master branch and the great LearnDB.
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