Astarion, the Ascended

Ward—Pay 3 life.
Whenever you gain life, Astarion explores.
Then destroy all other creatures if it's power
is exactly 20. (To have this creature explore, reveal the top card of your library. Put that
card into your hand if it's a land. Otherwise,
put a +1/+1 counter on this creature, then put
the card back or put it into your graveyard. )