It's On! is an Epic Spell introduced into Boss Monster: The Next Level.
It's On! is a card that is decent for killing an Ordinary Hero early on in order for preventing a wound, in case they may survive.
Alternatively, it can be saved during the late game when Epic Heroes come to town, as the player will have more likely have acquired 3 wounds by that time, and this card can kill Epic Heroes with ease.
The artwork features an enraged Killa.
The name of the spell most likely comes from the saying "It's on like Donkey Kong!", which is essentially another way of saying "Bring it on". Which, the saying itself makes a reference to the Nintendo character, Donkey Kong.