Idea for Varus if he ever gets an update.
When his passive procs on champion kills he goes fully Darkin. But when it goes down, he returns to a normal state.
Or maybe each kill he gets would make him a little bit more Darkin-like and when he gets a pentakill he's fully Darkin.
Edit: I mean in my second suggestion he gradually becomes more Darkin-like with each kill leading up to a penta, but it gradually goes back down again.