Mass of Remains's power is equal to the total number of permanent cards you own in your graveyard and in exile.
Spellcharge 3

Target creature you control gets +2/+1 until end of turn. It fights up to one target creature you don't control.

Devoid
Target creature gains double strike until end of turn. Put a +1/+1 counter on that creature if it's colorless.

When Maven of Fear enters, mill two cards.
Maven of Fear gets +2/+1 and has menace as long as six or more cards are in your graveyard.

Whenever another creature you control enters, Mayuk Claw-Sage gets +1/+0 and gains trample until end of turn.

Target creature gains indestructible until end of turn. Teach it lifelink.

When Meeting of the Elites enters, create a tapped Treasure token.
You've sacrificed a nonland permanent this turn.

When this creature enters, create X green Subroutine enchantment tokens named MOUNT with “Whenever this Subroutine runs, target creature you control gets +2/+2 until end of turn.”

Whenever you draw your second card each turn, scry 1 then put a +1/+1 counter on Melancholic Oracle.

{T}: Target player mills two cards.
{2}, {T}: Target player mills four cards.
{4}, {T}: Draw two cards. Activate this ability only if a player has ten or more cards in their graveyard.

{T}: Target player mills two cards.
{T}: Add one mana of any color. Spend this mana only to cast spells from your graveyard or activate abilities of cards in your graveyard.

When Menelaia enters, put a +1/+1 counter on up to two target creatures.
{1}, Remove a +1/+1 counter from another creature you control: That creature gain indestructible until end of turn. Tap it.

{1}{U/R}, {T}: Tap target creature, then gain control of it until end of turn. Activate only as a sorcery.

{W}, {T}: Tap target creature. Activate only as a sorcery.
{6}{W}{W}, {T}: Tap all creatures you don't control. Activate only as a sorcery.

Harness
Exile the top three cards of your library. Cast a spell with mana value X or less from among them without paying its mana cost.

Mev can't be blocked by creatures with greater power.
Whenever Mev attacks, draw a card.
Whenever Mev deals damage to a player, that player draws a card.

Exile the top three cards of your library. You may play up to two of those cards this turn.

Dragon spells and Rift spells you cast cost {1} less to cast.

{R}, {T}: Copy target activated ability you control. You may choose new targets for the copy.

Whenever you cast a noncreature spell, surveil 1.
If you would draw a card while your library has no cards in it, you may instead put a card you own from exile or outside the game into your hand.

{T}: Add {G}.
{T}: Add {G}{G}. You can't spend this mana to cast spells.

Ninjutsu {G}
Whenever this creature deals combat damage to a player, put a +1/+1 counter on target creature you control that doesn't have a +1/+1 counter on it.

Return target creature or enchantment to its owner's hand. If that permanent's controller has eight or more cards in their graveyard, its owner puts it on the top or bottom of their library instead.

Surveil 2, then choose an instant or sorcery card in your graveyard. You may cast it this turn. If that spell would be put into your graveyard this turn, exile it instead.

Whenever Mistcall Mariner becomes tapped, up to one target creature gets -2/-0 until end of turn.