msem-instigator

When Mountain-Western Inventor enters, you may return target artifact card from your graveyard to your hand.
{1}, Sacrifice an artifact: Target creature can't block or be blocked this turn.

Trample
Whenever Quakestep Behemoth or another creature you control enters, put X +1/+1 counters on each creature you control, where X is the entering creature's power.

Flying

Reach
Attain {4}{G} ({4}{G}: Transform this. Spend at least three colors of mana to activate this, and only as a sorcery.)

Look at the top three cards of your library. Put one on top and the rest on the bottom in any order.
Draw two cards. You lose 2 life.

Colossal 2—{6}{W}{W} (You may cast this for its colossal cost to have it enter with two +1/+1 counters; or pay {6}{W}{W} as a sorcery: Put two +1/+1 counters on it if it has none.)
When Heroes' Adversary's colossal ability puts one or more counters on it, exile target creature an opponent controls until Heroes' Adversary leaves the battlefield.

Menace
Whenever a nontoken creature you don't control dies, create a token that's a copy of that creature except it's a colorless Spirit. Exile it at the beginning of your next end step.

Whenever Adept Lieutenant attacks, 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.”)
{1}, Sacrifice an Idol: Put a +1/+1 counter on Adept Lieutenant.

Harmony — Scry 2.
{T}: Harmonize. Activate only as a sorcery. (Trigger up to one harmony of a permanent you control.)

When Savage Savant enters, create a 4/4 green Beast creature token.

This spell costs {X} less to cast, where X is 20 minus your life total.
First strike, menace, trample

When Journey to the Otherworlds enters, draw a card, then choose a creature card name.
Whenever a creature you control with the chosen name attacks, you may aetherize it. (To aetherize, create a token that's a copy of it, then exile that token.)

Whenever another artifact is put into your graveyard from the battlefield, each opponent loses 1 life and you gain 1 life.

Ward {2} (Whenever this creature becomes the target of a spell or ability an opponent controls, counter it unless that player pays {2}.)
When Sea's Allure enters, gain control of target creature for as long Sea's Allure remains on the battlefield.

Each player sacrifices a creature.
Coalesce {5}{B} (You may cast this from your graveyard for {5}{B}. If you do, it enters as a Horror creature after it resolves. Exile it if it would leave the battlefield or not resolve.)

Enchant opponent
Whenever you attack enchanted player, look at the top X cards of your library, where X is one plus the number of attacking creatures you control. Put one of those cards into your hand and the rest on the bottom of your library in any order.

Search your library for a basic land card, reveal it, put it into your hand, then shuffle.
Compose (Then if you cast this from your hand, exile it and create a Saga token with “r3—You may cast a copy of this card from exile without paying its mana cost.”)

Gestalt (When this or another creature enters under your control, you may merge it with the one in play, keeping counters and attachments. They become the permanent on top plus all abilities from under it.)
This creature gets +2/+2 and has trample as long as it's merged with another card or token.

Puzzlebox (Begin the game with this and another card you own from outside the game in the command zone, locked. That card is face down until you unlock this.)
{5}: Unlock Key to the Nexus. If {W}{U}{B}{R}{G} was spent to activate this ability, you may cast the revealed card without paying its mana cost. Otherwise, put that card on top of your library.

{7}{U}: Tap up to three target nonland permanents. This ability costs {2} less to activate if you control a legendary creature.

If a spell or ability would cause a player to discard one or more cards, that player discards that many cards at random instead.
Whenever a player discards a card, that player loses 2 life.

Flash
When Dragonwall Samurai enters, prevent all combat damage target creature would deal this turn.

Siphon (Each -1/-1 counter you put on your creatures up to their remaining toughness while casting this pays for {1}.)
Search your library for up to two Forest cards, reveal those cards, put one onto the battlefield tapped and the other into your hand, then shuffle.

This spell costs {X} less to cast, where X is the greatest power among creatures you control.
Tap up to three target creatures. They don't untap during their controller's next untap step.

[+2]: Until end of turn, you may spend mana as though it were mana of any type.
[0]: For each color, put a +1/+1 counter on a creature you control of that color.
[–7]: For each color, search your library for up to one creature card of that color. Put those cards onto the battlefield, then shuffle.