When Mairsil enters, you may exile an artifact or creature card from your hand or graveyard and put a cage counter on it. Mairsil has all activated abilities of all cards you own in exile with cage counters on them. You may activate each of those abilities only once each turn.

Defender {T}: Exchange target opponent's life total with this creature's toughness.

Until end of turn, target creature loses all abilities and becomes a blue Frog with base power and toughness 1/1.

Search your library for a card, put that card into your graveyard, then shuffle.

Wither (This deals damage to creatures in the form of -1/-1 counters.) {2}{R}, {Q}: This creature deals damage equal to its power to any target. ({Q} is the untap symbol.)