Sidra, Dragon Of The Stars

Flying, Trample, Astral (When this creature attacks, create a star counter)
When this card enters the battlefield, create X
star counters. When you run out of star counters,
sacrifice this creature.
When this card attacks, You may sacrificee one
star counter. if you do, flip a coin. If heads, you
may search your library for a card with {r} or {w} in their mana cost, reveal it, and put it into
your hand. if tails, Target opponent puts the top
card of their library into their graveyard