Until end of turn, target creature you control gains indestructible and "Whenever this creature is dealt damage, put that many +1/+1 counters on it."

: Untap another target snow permanent.

Whenever another Merfolk you control enters, put a +1/+1 counter on target Merfolk you control.

When this creature enters, return a black or red creature you control to its owner's hand.
When this creature enters, each player sacrifices a land of their choice.

: Add . If you control an Urza's Power-Plant and an Urza's Tower, add instead.

: Mill two cards. (Put the top two cards of your library into your graveyard.)
, Exile this artifact: Shuffle any number of target cards from your graveyard into your library.

Sliver creatures you control have "When this creature enters, you may exile target creature an opponent controls until this creature leaves the battlefield."

When this creature enters, any opponent may have it deal 4 damage to them. If a player does, sacrifice this creature.

Reach
At the beginning of your first main phase, add for each creature you control with power 4 or greater.

Menace
When this creature enters or dies, exile another card from a graveyard. When you do, choose one —
• Remove X counters from target permanent, where X is the mana value of the exiled card.
• Target creature an opponent controls gets -X/-X until end of turn, where X is the mana value of the exiled card.

Convoke (Your creatures can help cast this spell. Each creature you tap while casting this spell pays for or one mana of that creature's color.)
Choose a creature type. Draw a card for each permanent you control of that type.

Search your library for up to four land cards with different names and reveal them. An opponent chooses two of those cards. Put the chosen cards into your graveyard and the rest onto the battlefield tapped, then shuffle.

: Add .
, , Sacrifice this land: Draw a card. Activate only if you control five or more lands.

Reach, trample, indestructible
Bast can't attack or block unless you control three or more creatures.
Whenever you attack, target attacking creature gets +X/+X until end of turn, where X is the number of creatures you control.

Activated abilities of nontoken Rebels cost an additional "Sacrifice a land" to activate.

Target creature you control gets +X/+X until end of turn. Then it fights up to one target creature you don't control. (Each deals damage equal to its power to the other.)

When this creature enters, you may search your library for an Aura card, reveal it, put it into your hand, then shuffle.

Instant and sorcery spells you control have lifelink.
At the beginning of your upkeep, exile an instant or sorcery card at random from your graveyard. You may cast it this turn. If a spell cast this way would be put into your graveyard, exile it instead.

When this creature enters, target creature gets +1/+1 until end of turn.

Prototype — 2/1 (You may cast this spell with different mana cost, color, and size. It keeps its abilities and types.)
When this creature enters, if you cast it, exile target instant or sorcery card with mana value less than or equal to this creature's power from your graveyard. Copy that card. You may cast the copy without paying its mana cost.