Does he rap with roots, or trap with roots?

This ability is cool, but the rules make it rather fiddly. In any case it needs "equal to its power" adding. Ideally it'd need to say "gains wither UEOT and deals damage to itself equal to its power", or perhaps "gains wither, deals damage etc, then loses wither".

Or "deals damage to itself equal to its power as if it had wither."

Also, it plainly traps 'roos, Bruce.

Hm, yes, both of those suggestions are superior to mine :)

Although, frankly, is there anything wrong with "Put a number of -1/-1 counters equal to target creature's power onto that creature"?