Swimmer in Nightmares

a Nightmare enters the battlefield — 151 cards in the game can cause this (Swimmer in Nightmares is one).

1. Umbris, Fear Manifest — When Swimmer in Nightmares enters, Umbris, Fear Manifest exiles a card from an opponent's library

a Merfolk deals combat damage — 386 cards in the game can cause this (Swimmer in Nightmares is one).

1. Seafloor Oracle — When Swimmer in Nightmares deals combat damage, Seafloor Oracle draws you 1 card

a Merfolk enters the battlefield — 397 cards in the game can cause this (Swimmer in Nightmares is one).

When Swimmer in Nightmares enters:

1. Forerunner of the Heralds — puts a +1/+1 counter on itself
2. Deeproot Elite — puts a counter on a Merfolk
3. River Sneak — gets +1/+1

anything attacks — 17,934 cards in the game can cause this (Swimmer in Nightmares is one).

When Swimmer in Nightmares attacks:

1. Far Fortune, End Boss — deals 1 damage
2. Temmet, Naktamun's Will — draws you 1 card
3. Grim Javelineer — gives +1/+0
4. Invisible Woman — triggers
5. Caesar, Legion's Emperor — sacrifices a creature
6. Guide of Souls — triggers
7. Campsite Cuisine — sacrifices a Food
8. Gossip's Talent — triggers
9. Banon, the Returners' Leader — draws and discards
10. Toph, Earthbending Master — puts a counter on a permanent

112 other cards care about it too. These are the ones with the most connections.

a creature attacks — 17,934 cards in the game can cause this (Swimmer in Nightmares is one).

When Swimmer in Nightmares attacks:

1. Adaptive Omnitool — searches a library
2. Anthem of Rakdos — gives +2/+0
3. Kindred Discovery — draws you 1 card
4. Invasion of New Capenna // Holy Frazzle-Cannon — puts counters on a permanent
5. Chorale of the Void — brings a card back
6. Anim Pakal, Thousandth Moon — puts a +1/+1 counter on itself
7. Saw — sacrifices a creature
8. Gisa's Favorite Shovel — makes an opponent sacrifice a creature
9. Ultima Weapon — kills a creature an opponent controls
10. Total War — kills a creature

180 other cards care about it too. These are the ones with the most connections.

a creature blocks — 17,934 cards in the game can cause this (Swimmer in Nightmares is one).

When Swimmer in Nightmares blocks:

1. Mage Hunters' Onslaught — makes a player lose 1 life
2. Carnage Gladiator — makes a player lose 1 life
3. Barrow-Blade — strips abilities
4. Heat of Battle — deals 1 damage
5. Battle Strain — deals 1 damage
6. Favor of the Woods — gains you 3 life
7. Tide of War — sacrifices a creature
8. Dead-Iron Sledge — kills a creature
9. Dwindle — kills a creature
10. Shield of the Righteous — triggers

18 other cards care about it too. These are the ones with the most connections.

a nontoken creature attacks — 17,934 cards in the game can cause this (Swimmer in Nightmares is one).

When Swimmer in Nightmares attacks:

1. Mardu Ascendancy — makes a token
2. Basri Ket — makes a token
3. Militia's Pride — makes a token

a creature deals combat damage — 17,934 cards in the game can cause this (Swimmer in Nightmares is one).

When Swimmer in Nightmares deals combat damage:

1. Locutus of Borg — makes a token
2. Antagonist Incorporator — makes a token
3. Hive Mind Coprocessor — makes you lose life

195 other cards care about it too. These are the ones with the most connections.

a creature is cast — 18,010 cards in the game can cause this (Swimmer in Nightmares is one).

When Swimmer in Nightmares is cast:

1. Primeval Bounty — makes a token
2. Bloodbond March — brings a card back
3. Dancing from Dark to Dawn — puts that spell's mana value counters on a creature
4. Endrek Sahr, Master Breeder — makes that spell's mana value tokens
5. Summon: Fenrir — it arrives with counters thanks to Summon: Fenrir
6. Summon: Brynhildr — grants haste
7. Skittering Horror — sacrifices itself
8. Skittering Skirge — sacrifices itself
9. Skittering Monstrosity — sacrifices itself
10. Silverback Elder — triggers

81 other cards care about it too. These are the ones with the most connections.

a nontoken creature enters the battlefield — 18,448 cards in the game can cause this (Swimmer in Nightmares is one).

When Swimmer in Nightmares enters:

1. Riku of Two Reflections — makes a token
2. Samwise Gamgee — makes a token

31 other cards care about it too. These are the ones with the most connections.

a creature enters the battlefield — 19,800 cards in the game can cause this (Swimmer in Nightmares is one).

When Swimmer in Nightmares enters:

1. Slash, Reptile Rampager — deals 2 damage
2. Ray Fillet, Wave Warrior — puts a counter on itself
3. The Neutrinos — gets +1/+0
4. Wingnut, Bat on the Belfry — grants flying, haste and menace
5. Aetherstorm Roc — puts a counter on a permanent you control
6. Raphael, Most Attitude — exiles a card from your library

300 other cards care about it too. These are the ones with the most connections.

anything enters the battlefield — 26,499 cards in the game can cause this (Swimmer in Nightmares is one).

1. Prowl, Stoic Strategist // Prowl, Pursuit Vehicle — When Swimmer in Nightmares enters, Prowl, Stoic Strategist // Prowl, Pursuit Vehicle puts a +1/+1 counter on itself

39 other cards care about it too. These are the ones with the most connections.