Elementalist's Palette

Whenever you cast a spell with {X} in its mana cost, put two charge counters on this artifact.
{T}: Add one mana of any color.
{T}: Add {C} for each charge counter on this artifact. Spend this mana only on costs that contain {X}.

Lifeblood Hydra

Trample
This creature enters with X +1/+1 counters on it.
When this creature dies, you gain life and draw cards equal to its power.

Spell Burst

Buyback {3} (You may pay an additional {3} as you cast this spell. If you do, put this card into your hand as it resolves.)
Counter target spell with mana value X.

Gadwick, the Wizened

When Gadwick enters, draw X cards.
Whenever you cast a blue spell, tap target nonland permanent an opponent controls.

Up the Beanstalk

When this enchantment enters and whenever you cast a spell with mana value 5 or greater, draw a card.

Stonecoil Serpent

Reach, trample, protection from multicolored
This creature enters with X +1/+1 counters on it.

Outcaster Trailblazer

When this creature enters, add one mana of any color.
Whenever another creature you control with power 4 or greater enters, draw a card.
Plot {2}{G} (You may pay {2}{G} and exile this card from your hand. Cast it as a sorcery on a later turn without paying its mana cost. Plot only as a sorcery.)

Gargos, Vicious Watcher

Vigilance
Hydra spells you cast cost {4} less to cast.
Whenever a creature you control becomes the target of a spell, Gargos fights up to one target creature you don't control.

Retribution of the Ancients

{B}, Remove X +1/+1 counters from among creatures you control: Target creature gets -X/-X until end of turn.

Skyshroud Claim

Search your library for up to two Forest cards, put them onto the battlefield, then shuffle.

Nissa, Steward of Elements

+2: Scry 2.
0: Look at the top card of your library. If it's a land card or a creature card with mana value less than or equal to the number of loyalty counters on Nissa, you may put that card onto the battlefield.
−6: Untap up to two target lands you control. They become 5/5 Elemental creatures with flying and haste until end of turn. They're still lands.

Hydroid Krasis

When you cast this spell, you gain half X life and draw half X cards. Round down each time.
Flying, trample
This creature enters with X +1/+1 counters on it.

Prowling Serpopard

This spell can't be countered.
Creature spells you control can't be countered.

Nexos

Strategic Coordinator — Basic lands you control have "{T}: Add {C}{C}. Spend this mana only on costs that contain {X}."

Villainous Wealth

Target opponent exiles the top X cards of their library. You may cast any number of spells with mana value X or less from among them without paying their mana costs.

Finale of Revelation

Draw X cards. If X is 10 or more, instead shuffle your graveyard into your library, draw X cards, untap up to five lands, and you have no maximum hand size for the rest of the game.
Exile Finale of Revelation.

Altered Ego

This spell can't be countered.
You may have this creature enter as a copy of any creature on the battlefield, except it enters with X additional +1/+1 counters on it.

Zimone, Paradox Sculptor

At the beginning of combat on your turn, put a +1/+1 counter on each of up to two target creatures you control.
{G}{U}, {T}: Double the number of each kind of counter on up to two target creatures and/or artifacts you control.

Rampaging Yao Guai

Vigilance, trample
This creature enters with X +1/+1 counters on it.
When this creature enters, destroy any number of target artifacts and/or enchantments with total mana value X or less.

Wave Goodbye

Return each creature without a +1/+1 counter on it to its owner's hand.

Repulsive Mutation

Put X +1/+1 counters on target creature you control. Then counter up to one target spell unless its controller pays mana equal to the greatest power among creatures you control.