Zrathrx

Menace, Double strike
When Zrathrx enters the battlefield, find all
legendary slivers from the library and put them
in a random order on the top of your library.
Zrathrx is just a land, if you have three or more
other legendary slivers on the battlefield, Zrathrx
becomes a creature.
Pay 13 life: draw 5 cards, put a legendary sliver
card from your hand onto the battlefield.