Just a quick confirmation: Can I corrupt Monster of the Deep (Action: Corrupt this unit and choose up to two target units. Those units cannot attack or defend until the end of the turn.) after having declared it as an attacker and before the opponent declares the defenders, thereby attacking with it will having fewer defenders to face? I suppose I can since corruption will only affect undeclared units.

Yes, you can do that. Corruption only prevents the unit to be chosen as a participant of a battle. If it is already attacking, Corrupting it will not hinder its ability to fight.

Thanks! That's very useful, but still this is a very expensive card...