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 at the end of your end step.)

At the beginning of your end step, if creatures you control have a total of 4 or more Hubris counters among them, you may transform Riblee, Shadowed Mentor.