msem-instigator

[+1]: Untap target land.
[–1]: Exile the top card of your library. You may play that card until end of turn. If a land card is exiled this way, you gain 2 life.
[–7]: Untap each land you control. Lands you control become 3/3 Elemental creatures with haste until end of turn. They're still lands.

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.

{U}{B}, {T}: Draw a card, then each player exiles the top card of their library. Then if a player owns two or more cards exiled with Arina, Volarian Explorer, exile her, then return her to the battlefield transformed under her owner's control.

When Arisen Haploc enters, return target creature card from a graveyard to its owner's hand.
Revive {5}{B}{B} ({5}{B}{B}, Exile this card from your graveyard: Create a token that's a copy of it except it has no mana cost. Revive only as a sorcery.)

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

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

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.”)

Whenever an artifact or enchantment you control enters, Armament Scavenger gets +1/+0 and gains trample until end of turn.

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.

Whenever two or more creatures an opponent controls attack, adjudicate. (Create a white Aura enchantment token with enchant creature and “Sacrifice this token: Untap enchanted creature” attached to target creature.)
Enchanted creatures you control get +0/+2.

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.”)