Thurm, the Contemplative

Hexproof
At the beginning of your end step, your
opponent gets 10 seconds to ask you a riddle.
If they don't, put a +1/+1 counter on Thurm.
If they do, at the beginning of the next end step,
you may offer an answer. If you do, your
opponent must tell you the correct answer. If
the answers match, put four +1/+1 counters
on Thurm.