Nightmare Incursion

a library is searched — 982 cards in the game can cause this (Nightmare Incursion is one).

When a permanent searches a library thanks to Nightmare Incursion:

1. River Song — puts a +1/+1 counter on itself
2. Archivist of Oghma — gains you 1 life
3. Wan Shi Tong, Librarian — puts a +1/+1 counter on itself
4. Ob Nixilis, Unshackled — makes an opponent sacrifice a creature

anything is exiled — 2,497 cards in the game can cause this (Nightmare Incursion is one).

1. The War Doctor — When a permanent is exiled thanks to Nightmare Incursion, The War Doctor puts a time counter on itself

a sorcery is cast — 3,720 cards in the game can cause this (Nightmare Incursion is one).

When Nightmare Incursion is cast:

1. Spellweaver Volute — copies a spell
2. Lilah, Undefeated Slickshot — gets +1/+1
3. Niblis of Frost — gets +1/+1
4. Primeval Bounty — puts 3 counters on a creature
5. Tetsuko Umezawa, Pursuer — gets +1/+1
6. Thor, God of Thunder — deals that spell's mana value damage
7. Krile Baldesion — brings a card back
8. Summon: Good King Mog XII — makes a token
9. Kykar, Wind's Fury — makes a token
10. Blood Funnel — sacrifices a creature

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

anything is cast — 31,128 cards in the game can cause this (Nightmare Incursion is one).

When Nightmare Incursion is cast:

1. Windrider Wizard — draws you 1 card
2. Relic Amulet — puts a charge counter on itself
3. Rockslide Sorcerer — deals 1 damage
4. Umara Wizard // Umara Skyfalls — grants flying
5. Alania, Divergent Storm — makes an opponent draw 1 card
6. Umara Mystic — gets +2/+0
7. Master of Winds — triggers

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