If you mean an arrow like move with crit, make the arrow drain mana like normal and add the critical strike in a hidden spell book, remove the hidden spell book on deactivation.
If you mean a move where by turning it on you gain a chance to crit but constantly drains mana, you do the same as the above but you need to check mana and if it is not enough, remove the spell book as well.
Hidden spell book allows you to use passives like auras, evasions, critical strike and bash withou their icon being visable, thus making it seem to be part of another spell.
To do it have the pasive spell inside a spell book ability, add the spell book ability and disable it (not remove).