The Mimeoplasm

As The Mimeoplasm enters, you may exile two creature cards from graveyards. If you do, it enters as a copy of one of those cards with a number of additional +1/+1 counters on it equal to the power of the other card.

Counterspell

Fierce Guardianship

If you control a commander, you may cast this spell without paying its mana cost.
Counter target noncreature spell.

Arcane Signet

{T}: Add one mana of any color in your commander's color identity.

Arcane Denial

Counter target spell. Its controller may draw up to two cards at the beginning of the next turn's upkeep.
You draw a card at the beginning of the next turn's upkeep.

Diabolic Tutor

Search your library for a card, put that card into your hand, then shuffle.

Seedship Broodtender

When this creature enters, mill three cards. (Put the top three cards of your library into your graveyard.)
{3}{B}{G}, Sacrifice this creature: Return target creature or Spacecraft card from your graveyard to the battlefield. Activate only as a sorcery.

Sol Ring

An Offer You Can't Refuse

Counter target noncreature spell. Its controller creates two Treasure tokens. (They're artifacts with "{T}, Sacrifice this token: Add one mana of any color.")

Voidslime

Counter target spell, activated ability, or triggered ability. (Mana abilities can't be targeted.)

Negate

Singularity Rupture

Destroy all creatures, then any number of target players each mill half their library, rounded down.

Demonic Tutor

Buried Alive

Search your library for up to three creature cards, put them into your graveyard, then shuffle.