Haste (This creature can attack, and as soon as it comes under your control.)
Whenever a creature an opponent controls dies, put a +1/+1 counter on Yahenni.
Sacrifice another creature: Yahenni gains indestructible (Effects that say "destroy" don't destroy that permanent. An indestructible creature can't be destroyed by damage.) until end of turn.
Vintage
Standard
and 
as soon as it comes under your control.)