Aetherize

Return all attacking creatures to their owner's hand.

Jin-Gitaxias, Progress Tyrant

Whenever you cast an artifact, instant, or sorcery spell, copy that spell. You may choose new targets for the copy. This ability triggers only once each turn. (A copy of a permanent spell becomes a token.)
Whenever an opponent casts an artifact, instant, or sorcery spell, counter that spell. This ability triggers only once each turn.

Arcane Subtraction

Target creature gets -4/-0 until end of turn.
Learn. (You may reveal a Lesson card you own from outside the game and put it into your hand, or discard a card to draw a card.)

Arcane Signet

{T}: Add one mana of any color in your commander's color identity.

Aether Tide

As an additional cost to cast this spell, discard X creature cards.
Return X target creatures to their owners' hands.

Amulet of Vigor

Whenever a permanent you control enters tapped, untap it.

Aethersnatch

Gain control of target spell. You may choose new targets for it. (If that spell becomes a permanent, it enters under your control.)

Amass the Components

Draw three cards, then put a card from your hand on the bottom of your library.

Altar of the Goyf

Whenever a creature you control attacks alone, it gets +X/+X until end of turn, where X is the number of card types among cards in all graveyards.
Lhurgoyf creatures you control have trample.

Ancestral Memories

Look at the top seven cards of your library. Put two of them into your hand and the rest into your graveyard.