Scurry of Squirrels

Myriad, myriad (Whenever this creature attacks, for each opponent other than defending player, you may create a token that's a copy of this creature that's tapped and attacking that player or a planeswalker they control. Then do it again. Exile the tokens at end of combat.)
Whenever this creature deals combat damage to a player, put a +1/+1 counter on target creature you control.

Arachnogenesis

Create X 1/2 green Spider creature tokens with reach, where X is the number of creatures attacking you. Prevent all combat damage that would be dealt this turn by non-Spider creatures.

Rikku, Resourceful Guardian

Whenever you put one or more counters on a creature, until end of turn, that creature can't be blocked by creatures your opponents control.
Steal — {1}, {T}: Move a counter from target creature an opponent controls onto target creature you control. Activate only as a sorcery.

Honest Work

Enchant creature an opponent controls
When this Aura enters, tap enchanted creature and remove all counters from it.
Enchanted creature loses all abilities and is a Citizen with base power and toughness 1/1 and "{T}: Add {C}" named Humble Merchant. (It loses all other creature types and names.)

Katara, Waterbending Master

Whenever you cast a spell during an opponent's turn, you get an experience counter.
Whenever Katara attacks, you may draw a card for each experience counter you have. If you do, discard a card.

Dress Down

Flash
When this enchantment enters, draw a card.
Creatures lose all abilities.
At the beginning of the end step, sacrifice this enchantment.

Mutational Advantage

Permanents you control with counters on them gain hexproof and indestructible until end of turn. Prevent all damage that would be dealt to those permanents this turn. Proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Oketra's Monument

White creature spells you cast cost {1} less to cast.
Whenever you cast a creature spell, create a 1/1 white Warrior creature token with vigilance.

Twitching Doll

{T}: Add one mana of any color. Put a nest counter on this creature.
{T}, Sacrifice this creature: Create a 2/2 green Spider creature token with reach for each counter on this creature. Activate only as a sorcery.

Marwyn, the Nurturer

Whenever another Elf you control enters, put a +1/+1 counter on Marwyn.
{T}: Add an amount of {G} equal to Marwyn's power.

Assassin Den

Defender (This creature can't attack.)
{3}{U}: Put a +1/+1 counter on target creature you control. It can't be blocked this turn. Activate only as a sorcery.

Blitzball Stadium

When this artifact enters, support X. (Put a +1/+1 counter on each of up to X target creatures.)
Go for the Goal! — {3}, {T}: Until end of turn, target creature gains "Whenever this creature deals combat damage to a player, draw a card for each kind of counter on it" and it can't be blocked this turn.

Hornet Nest

Defender (This creature can't attack.)
Whenever this creature is dealt damage, create that many 1/1 green Insect creature tokens with flying and deathtouch. (Any amount of damage a creature with deathtouch deals to a creature is enough to destroy it.)

Herald of Secret Streams

Creatures you control with +1/+1 counters on them can't be blocked.

Ezuri, Claw of Progress

Whenever a creature you control with power 2 or less enters, you get an experience counter.
At the beginning of combat on your turn, put X +1/+1 counters on another target creature you control, where X is the number of experience counters you have.