msem-instigator

[+1]: Search your library for a land card, put it onto the battlefield, untap it, then shuffle. Sacrifice a land.
[–2]: Destroy target noncreature permanent. Its controller may search their library for a basic land card, put that card onto the battlefield, then shuffle.
[–6]: Return any number of land cards from your graveyard to the battlefield and untap them. They become 3/3 creatures with haste in addition to their other types.

Whenever Arena Challenger deals damage to a creature, upgrade it. (Give it one of the listed counters it doesn't already have.)
• +1/+1
• First strike
• Prowess

Arid Causeway enters tapped unless you control another Desert.
{T}: Add {R} or {W}.

When Aridland Camaar enters, create a 1/1 white Bird creature token with flying.

If a creature would leave the battlefield this turn, exile it instead.
Spell mastery — If there are two or more instant and/or sorcery cards in your graveyard, target player sacrifices a creature.

Whenever Arisen Helix attacks, scry 2.
If Arisen Helix would die, exile it instead.

Haste
Whenever Arisen Hunter attacks, target player loses 2 life and you gain 2 life.
If Arisen Hunter would die, exile it instead.

Whenever you cast a noncreature spell, each opponent loses 1 life.
{4}, Exile Arisen Psion from your graveyard: Draw a card.

Whenever Arisen Rider attacks, creatures your opponents control get -1/-1 until end of turn.
If Arisen Rider or another creature or planeswalker would die, exile it instead.

When Arisen Troops dies, you may destroy target creature that was dealt damage this turn.

Trample
Whenever Arisen Tyrant attacks, mill three cards, then you may exile a card from your graveyard. You may cast a copy of the exiled card without paying its mana cost if the spell's mana value is 3 or less.
If Arisen Tyrant would die, exile it instead.

Aristocrat's Gallery enters tapped.
When Aristocrat's Gallery enters, look at the top card of your library. You may put it into your graveyard.
{T}: Add {G} or {U}.

Menace, haste
As long as Arleigh, Spellblade Prodigy is tapped, the first spell you cast during each other player's turn costs {U/R} less to cast. This effect reduces only the amount of colored mana you pay. (For example, if you cast a spell with mana cost {1}{R}, it costs {1} to cast.)

Flying
Whenever a creature you control phases in, it gains flying until end of turn.

Enchant creature
Enchanted creature gets +2/+2 and has trample.
Whenever enchanted creature deals damage, you gain that much life.

Target creature you control gets +2/+2 until end of turn. If it's attacking, create an Idol token. (It's an artifact with “{1}, Sacrifice this artifact: Put a +1/+1 counter on target creature. Activate only as a sorcery.”)

Target creature gains double strike until end of turn.

As long as Armed Vendor is enchanted or equipped, it gets +1/+0 and has menace.

Enchant creature (Target a creature as you cast this. This card enters attached to that creature.)
Enchanted creature gets +2/+2.
{1}{G}: Regenerate enchanted creature.

Prevent all combat damage that would be dealt to Armored Axewarden as long as it's embattled. (A creature is embattled if it has a counter on it or a permanent you control attached to it.)

Reach, vigilance

At the beginning of your end step, you may put a progress counter on Armored Transports.
[PROGRESS 1-2] Creatures you control get +0/+1.
[PROGRESS 3+] Creatures you control get +0/+1 and have vigilance.

When Armored Valiant enters, creatures you control get +1/+1 until end of turn.

When Arms Merchant enters, look at the top four cards of your library. You may reveal an artifact card from among them and put it into your hand. Put the rest on the bottom of your library in any order.

Prowess
When Arms Tinkerer enters, choose one —
• Destroy target artifact.
• Create a Treasure token. (It's an artifact with “{T}, Sacrifice this artifact: Add one mana of any color.”)