Plague Mare

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

1. Umbris, Fear Manifest — When Plague Mare enters, Umbris, Fear Manifest exiles a card from an opponent's library

a creature in a graveyard — 2,769 cards in the game can cause this (Plague Mare is one).

Plague Mare puts cards into the graveyard that:

1. Chainer, Dementia Master — can bring back
2. The Deck of Many Things — can bring back
3. Afterlife from the Loam — can bring back
4. Crypt Champion — can bring back
5. Victor, Valgavoth's Seneschal — can bring back
6. Push // Pull — can bring back
7. Vivien on the Hunt — can bring back
8. Virtus's Maneuver — can bring back
9. Dredger's Insight — can bring back
10. Bramble Familiar // Fetch Quest — can bring back

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

a nontoken creature dies — 2,769 cards in the game can cause this (Plague Mare is one).

When a creature dies thanks to Plague Mare:

1. Vraska, the Silencer — brings a card back
2. High-Society Hunter — draws you 1 card
3. Overseer of the Damned — makes a token
4. Shelob, Dread Weaver — hits a graveyard
5. Ghoulish Procession — makes a token
6. Ogre Slumlord — makes a token
7. Minotaur, Roxxon CEO — makes a token
8. Harvester of Souls — draws you 1 card
9. Magus of the Bridge — exiles itself
10. Koh, the Face Stealer — exiles it

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

a creature dies — 2,830 cards in the game can cause this (Plague Mare is one).

When a creature dies thanks to Plague Mare:

1. Journey to Eternity // Atzal, Cave of Eternity — brings a card back
2. Avatar Destiny — mills a card
3. Blood Spatter Analysis — mills a card
4. Shadows of the Past — scries

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

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

When Plague Mare attacks:

1. Shaun, Father of Synths — makes a token
2. Surgical Suite // Hospital Room — puts a counter on a creature
3. Banon, the Returners' Leader — draws and discards

145 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 (Plague Mare is one).

1. Chorale of the Void — When Plague Mare attacks, Chorale of the Void brings a card back

219 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 (Plague Mare is one).

When Plague Mare enters:

1. Liliana the Repentant — mills a card
2. Pious Evangel // Wayward Disciple — gains you 1 life
3. Daxos, Blessed by the Sun — gains you 1 life

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