When you cast this spell, reveal the top X cards of your library. You may put a nonland permanent card with mana value X or less from among them onto the battlefield. Then shuffle the rest into your library.
Genesis Hydra enters with X +1/+1 counters on it.
If you put an Aura onto the battlefield this way, you choose what it enchants as it enters the battlefield. This doesn't target any permanent or player, but it must be able to enchant that permanent or player. For example, you could put a green Aura onto the battlefield enchanting a creature with hexproof controlled by an opponent, but not one with protection from green.
17-11-2017
wotc
A nonland permanent card is an artifact, creature, enchantment, or planeswalker card.
17-11-2017
wotc
If a card in a player's library has {X} in its mana cost, X is considered to be 0.
17-11-2017
wotc
If "the rest" is zero cards, either because X was 0 or because X was 1 and that card was put onto the battlefield, the library is still shuffled.
17-11-2017
wotc
Genesis Hydra's first ability will resolve before Genesis Hydra does. Notably, if you put an Aura card onto the battlefield this way, it can't enchant Genesis Hydra.
17-11-2017
wotc
If you have fewer than X cards in your library, you reveal all of them.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.