Nivix, Aerie of the Firemind

a land enters the battlefield — 1,560 cards in the game can cause this (Nivix is one).

When Nivix, Aerie of the Firemind enters:

1. Primeval Bounty — gains you 3 life
2. Toph, Earthbending Master — puts a counter on a permanent
3. Dancing from Dark to Dawn — makes a token
4. Emeria Shepherd — brings a card back
5. Quest for the Necropolis — puts a quest counter on itself
6. Soul Stair Expedition — puts a quest counter on itself
7. Nissa of Shadowed Boughs — puts counters on a permanent
8. Thranduil, Sindarin Liege // Silvan Rally — makes a token
9. Silvan Reveler — returns itself from the graveyard
10. Floral Evoker — puts a +1/+1 counter on itself

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

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

1. The War Doctor — When a permanent is exiled thanks to Nivix, Aerie of the Firemind, The War Doctor puts a time counter on itself

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

1. Spellweaver Volute — When a sorcery is cast thanks to Nivix, Aerie of the Firemind, Spellweaver Volute copies a spell
2. Riku of Two Reflections — When an instant is cast thanks to Nivix, Aerie of the Firemind, Riku of Two Reflections copies a spell
3. Sword of Wealth and Power — When an instant is cast thanks to Nivix, Aerie of the Firemind, Sword of Wealth and Power copies a spell
4. Lilah, Undefeated Slickshot — When an instant is cast thanks to Nivix, Aerie of the Firemind, Lilah, Undefeated Slickshot gets +1/+1
5. Extus, Oriq Overlord // Awaken the Blood Avatar — When an instant is cast thanks to Nivix, Aerie of the Firemind, Extus, Oriq Overlord // Awaken the Blood Avatar brings a card back
6. Metallurgic Summonings — When an instant is cast thanks to Nivix, Aerie of the Firemind, Metallurgic Summonings makes a token
7. Storm-Kiln Artist — When an instant is cast thanks to Nivix, Aerie of the Firemind, Storm-Kiln Artist makes a token
8. Niv-Mizzet, Parun — When an instant is cast thanks to Nivix, Aerie of the Firemind, Niv-Mizzet, Parun draws you 1 card
9. Niblis of Frost — When an instant is cast thanks to Nivix, Aerie of the Firemind, Niblis of Frost taps a creature an opponent controls
10. Mica, Reader of Ruins — When an instant is cast thanks to Nivix, Aerie of the Firemind, Mica, Reader of Ruins sacrifices an artifact

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