Riblee, Shadowed Mentor

Whenever one or more creatures you control attack, put a Hubris counter on each attacking creature. (Each Hubris counter gives a creature +1/+1. If a creature has 3 or more Hubris counters, sacrifice it during your end step.)

At the beginning of your end step, if there are 4 or more hubris counters among creatures you control, you may transform Riblee, Shadowed Mentor.