Command Undead does just what it sounds like it does. As a core ability of the Necromancer class, it allows the possessor to temporarily give instructions to undead he does not control already. The duration is very short, even at higher levels, so Command Undead is primarily useful as a combat ability, since the undead will turn on you again too soon to do much more than attack other undead. It affects a single target. It can be countered by an opposing Command Undead and as long as the durations overlap the undead returns to its state prior to being commanded.
Only Necromancers learn Command Undead, but fanatics of a death deity could easily get it, and it isn’t hard for someone who frequently deals with undead to get it cross-class.
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