Thirst for Discovery

Draw three cards. Then discard two cards unless you discard a basic land card.

Sludge Monster

Whenever Sludge Monster enters or attacks, put a slime counter on up to one other target creature.
Non-Horror creatures with slime counters on them lose all abilities and have base power and toughness 2/2.

Shipwreck Marsh

Shipwreck Marsh enters tapped unless you control two or more other lands.
{T}: Add {U} or {B}.

Krothuss, Lord of the Deep

Flying
Whenever Krothuss, Lord of the Deep attacks, create a tapped and attacking token that's a copy of another target attacking creature. If that creature is a Kraken, Leviathan, Octopus, or Serpent, create two of those tokens instead.

Runo Stromkirk

Flying
When Runo Stromkirk enters, put up to one target creature card from your graveyard on top of your library.
At the beginning of your upkeep, look at the top card of your library. You may reveal that card. If a creature card with mana value 6 or greater is revealed this way, transform Runo Stromkirk.

Runo Stromkirk // Krothuss, Lord of the Deep

Hullbreaker Horror

Flash
This spell can't be countered.
Whenever you cast a spell, choose up to one —
• Return target spell you don't control to its owner's hand.
• Return target nonland permanent to its owner's hand.

Infernal Grasp

Destroy target creature. You lose 2 life.

Consider

Surveil 1. (Look at the top card of your library. You may put it into your graveyard.)
Draw a card.

Toxrill, the Corrosive

At the beginning of each end step, put a slime counter on each creature you don't control.
Creatures you don't control get -1/-1 for each slime counter on them.
Whenever a creature you don't control with a slime counter on it dies, create a 1/1 black Slug creature token.
{U}{B}, Sacrifice a Slug: Draw a card.

Olag, Ludevic's Hubris

As this creature transforms into Olag, Ludevic's Hubris, it becomes a copy of a creature card exiled with it, except its name is Olag, Ludevic's Hubris, it's 4/4, and it's a legendary blue and black Zombie in addition to its other colors and types. Put a number of +1/+1 counters on Olag equal to the number of creature cards exiled with it.

Ludevic, Necrogenius

Whenever Ludevic, Necrogenius enters or attacks, mill a card.
{X}{U}{U}{B}{B}, Exile X creature cards from your graveyard: Transform Ludevic. X can't be 0. Activate only as a sorcery.

Ludevic, Necrogenius // Olag, Ludevic's Hubris

Ice Tunnel

({T}: Add {U} or {B}.)
Ice Tunnel enters tapped.

Snow-Covered Island

Jwari Ruins

Jwari Disruption

Counter target spell unless its controller pays {1}.

Jwari Disruption // Jwari Ruins

Icebreaker Kraken

This spell costs {1} less to cast for each snow land you control.
When Icebreaker Kraken enters, artifacts and creatures target opponent controls don't untap during that player's next untap step.
Return three snow lands you control to their owner's hand: Return Icebreaker Kraken to its owner's hand.

Murkwater Pathway

Clearwater Pathway

Clearwater Pathway // Murkwater Pathway

Power Word Kill

Destroy target non-Angel, non-Demon, non-Devil, non-Dragon creature.

Fading Hope

Return target creature to its owner's hand. If its mana value was 3 or less, scry 1. (Look at the top card of your library. You may put that card on the bottom.)

Snow-Covered Swamp

The Meathook Massacre

When The Meathook Massacre enters, each creature gets -X/-X until end of turn.
Whenever a creature you control dies, each opponent loses 1 life.
Whenever a creature an opponent controls dies, you gain 1 life.