Creatures only, according to this post:
http://magewars.com/jsite/forum/general-questions/896-mana-drain-vs-conjuration-objects
I think the second sentence about attacking multiple objects means that if you used Battle Fury (extra attack) on a mana drainer, you don't get any mana drain on the second attack, regardless of whether the first attack drained any mana or not.
Here is the wording again.
Mana Drain +X
If this attack damages an enemy creature, that creature’s controller loses
X mana from his mana supply (if he has any). If the attack makes multiple
attacks against the same or different objects,
it gains this bonus only for the
first attack it makes. Because of this last line that would only apply to the first attack regardless of result. The reason for this was in testing we used to give the Hydra mana drain and Battle Fury. Think about a Hyrda hitting with mana drain 2 for six attacks.
Also why triple strike looks like it does now. :blink: That was some good times.