Karim, Exotic Collector

{x}: Put a cage counter on target creature with converted mana cost X. Tap it. For as long as it
has a cage counter on it, it doesn't untap during
its controller's untap step.
At the beginning of your upkeep, you may gain control of target creature with a cage counter on
it. Untap it. Remove all cage counters from it.
"What an impressive creature, how much will it cost?"