Elder Brain

Flying

At the beginning of your end step, exile the top card of each opponent's library. You may play those cards for as long as they remain exiled. If you cast a spell this way, you may spend mana as though it were mana of any color to cast it.

Whenever a nonland permanent an opponent owns enters the battlefield under your control, they lose life equal to its mana value.

Arvinox isn't a creature unless you control three or more permanents you don't own.

At the beginning of your end step, exile the bottom card of each opponent's library face down. For as long as those cards remain exiled, you may look at them, you may cast permanent spells from among them, and you may spend mana as though it were mana of any color to cast those spells.

Trample

Whenever this creature deals combat damage to a player, you may look at that player's hand. If you do, you may cast a spell from among those cards without paying its mana cost.

Convoke

Flying

When this creature enters, search your library for a card, exile it face down, then shuffle. For as long as that card remains exiled, you may play it.

Spells you cast from exile have convoke.

Flying

Whenever this creature deals combat damage to a player, sacrifice all other permanents you control and discard your hand. Exile the top six cards of your library. You may cast any number of spells from among cards exiled this way without paying their mana costs.

Mutate (If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.)

Flying

Whenever this creature mutates, exile the top card of each opponent's library face down. You may look at and play those cards for as long as they remain exiled.