Overcharged Amalgam

Flash
Flying
Exploit (When this creature enters the battlefield, you may sacrifice a creature.)
When Overcharged Amalgam exploits a creature, counter target spell, activated ability, or triggered ability.

Flickerwisp

Flying
When Flickerwisp enters the battlefield, exile another target permanent. Return that card to the battlefield under its owner's control at the beginning of the next end step.

Acidic Slime

Deathtouch (Any amount of damage this deals to a creature is enough to destroy it.)
When Acidic Slime enters the battlefield, destroy target artifact, enchantment, or land.

Teleportation Circle

At the beginning of your end step, exile up to one target artifact or creature you control, then return that card to the battlefield under its owner's control.

Frilled Mystic

Flash
When Frilled Mystic enters the battlefield, you may counter target spell.

Auramancer

When Auramancer enters the battlefield, you may return target enchantment card from your graveyard to your hand.

Spara's Headquarters

({T}: Add {G}, {W}, or {U}.)
Spara's Headquarters enters the battlefield tapped.
Cycling {3} ({3}, Discard this card: Draw a card.)

Adrix and Nev, Twincasters

Ward {2} (Whenever this creature becomes the target of a spell or ability an opponent controls, counter it unless that player pays {2}.)
If one or more tokens would be created under your control, twice that many of those tokens are created instead.

Armada Wurm

Trample
When Armada Wurm enters the battlefield, create a 5/5 green Wurm creature token with trample.

Conjurer's Closet

At the beginning of your end step, you may exile target creature you control, then return that card to the battlefield under your control.

Magewright's Stone

{1}, {T}: Untap target creature that has an activated ability with {T} in its cost.

Draining Whelk

Flash
Flying
When Draining Whelk enters the battlefield, counter target spell. Put X +1/+1 counters on Draining Whelk, where X is that spell's mana value.

Avenger of Zendikar

When Avenger of Zendikar enters the battlefield, create a 0/1 green Plant creature token for each land you control.
Landfall — Whenever a land enters the battlefield under your control, you may put a +1/+1 counter on each Plant creature you control.

Aether Channeler

When Aether Channeler enters the battlefield, choose one —
• Create a 1/1 white Bird creature token with flying.
• Return another target nonland permanent to its owner's hand.
• Draw a card.

Storvald, Frost Giant Jarl

Ward {3}
Other creatures you control have ward {3}.
Whenever Storvald enters the battlefield or attacks, choose one or both —
• Target creature has base power and toughness 7/7 until end of turn.
• Target creature has base power and toughness 1/1 until end of turn.

Ghostly Flicker

Exile two target artifacts, creatures, and/or lands you control, then return those cards to the battlefield under your control.

Jedit Ojanen, Mercenary

Whenever Jedit Ojanen, Mercenary or another legendary creature enters the battlefield under your control, you may pay {G}. If you do, create a 2/2 green Cat Warrior creature token with forestwalk. (It can't be blocked as long as defending player controls a Forest.)

Agent of Treachery

When Agent of Treachery enters the battlefield, gain control of target permanent.
At the beginning of your end step, if you control three or more permanents you don't own, draw three cards.

Amareth, the Lustrous

Flying
Whenever another permanent enters the battlefield under your control, look at the top card of your library. If it shares a card type with that permanent, you may reveal that card and put it into your hand.

Falco Spara, Pactweaver

Flying, trample
Falco Spara, Pactweaver enters the battlefield with a shield counter on it.
You may look at the top card of your library any time.
You may cast spells from the top of your library by removing a counter from a creature you control in addition to paying their other costs.

Archaeomancer

When Archaeomancer enters the battlefield, return target instant or sorcery card from your graveyard to your hand.

Mystic Snake

Flash
When Mystic Snake enters the battlefield, counter target spell.

Soul of Emancipation

When Soul of Emancipation enters the battlefield, destroy up to three other target nonland permanents. For each of those permanents, its controller creates a 3/3 white Angel creature token with flying.