I disagree. We can kill things and eat their brainz memories to recover our power and feed our minions. So really, the only limit is that we can't be an evil lich, hiding in our tower, commanding self-replicating hordes of undead, we actually have to be near the front lines. Hmmm... Unless...
Command a skeleton to kill one of those test animals we purchased and then eat its memories. Then try to extract said memories from the skeleton or have the skeleton regurgitate them or something. Is such second-hand feeding possible?
If that works, then the next step is investigating phantom reproduction to see if we can split an overfed phantom into two and have one go to a reanimated corpse. That would probably require special "Commander" zombies that have necromancy rings.
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After a few tests the end result is this:
1. Once absorbed, memories cannot be voluntarily released, though you think you might be able to get them through damaging or killing the spirit in the same way you do via killing the living. However, how you would do that is something you haven't figured out.
2. The memories can be "Captured" by the spirits and then given to you instead of being fed upon. They can only "Hold" so much and they seem to degrade over time, but it can be done. However, only the crab animated skeleton seems bright enough and controlled enough to do so. The others are rather like dogs with fresh meat: You command them to leave it alone but their stomachs tend to win out in the debate. They're too simple and animal like.
On a related note: you can deactivate the spirit infused skeletons, the only issue would be that the way to do it is releasing the animating spirit, which would mean needing to grab a new one and do the animation process over again when you want them back. Which is of course somewhat tiring and wasteful.