I got to thinking that there really ought to be something you can do with Vengance Spirits, and this is what I came up with last night:.

1. Start with some way to simply bottle multiple VSs, and "throw" them at a target. This could be or lead to an item used to control the later developments, below.

2. Build a non-humanoid robot/golem, animated by a Vengance Spirit. It might heal slowly (perhaps by draining blood from mobs), or the player might need to supply blood to heal it. It can't be too strong (given that vengance spirits can be created easily enough), and it should have some liabilities -- perhaps it attacks everything (animals, villagers, pets?) not protected by a Burning Gem or Gem Torch, or has an effective range limit like dogs, or can't heal. Possible variations for the form:
   2a: Crawling thing, resembling a crab or scorpion, or worse.
   2b: Small flying thing, but that would really need either the range limit or a callback feature.
   2c: Rooted to a spot on the ground, or a short range nearby. This would be a territory guard.

3. While we're at it, install a Forgotten Player Spirit into a humanoid abomination, Much more powerful -- perhaps it can collect blood for its master, but also uses the same store for its own healing. This could be given armor and weapons as a fighter (it gets player drops), and might even be able to do mundane tasks similar to Thaumcraft golems (guard a location, collecting items, farming, inventory management). This one would probably include a BoEC in its creation, which would drop if it's killed, usually preserving the spirit.

Not really the direction I was going for with EC, but maybe some day.