Nix

an instant is cast — 3,931 cards in the game can cause this (Nix is one).

1. Lunar Mystic — When Nix is cast, Lunar Mystic draws you 1 card
2. Ojer Pakpatiq, Deepest Epoch // Temple of Cyclical Time — When Nix is cast, Ojer Pakpatiq, Deepest Epoch // Temple of Cyclical Time grants rebound
3. Repeated Reverberation — When Nix is cast, Repeated Reverberation copies an ability
4. Riku of Two Reflections — When Nix is cast, Riku of Two Reflections copies a spell
5. Sword of Wealth and Power — When Nix is cast, Sword of Wealth and Power copies a spell
6. Lilah, Undefeated Slickshot — When Nix is cast, Lilah, Undefeated Slickshot gets +1/+1
7. Gale, Waterdeep Prodigy — When Nix is cast, Gale, Waterdeep Prodigy brings a card back
8. Harness the Storm — When Nix is cast, Harness the Storm brings a card back
9. In the Eye of Chaos — When Nix is cast, In the Eye of Chaos counters a spell
10. Niblis of Frost — When Nix is cast, Niblis of Frost taps a creature an opponent controls

427 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 (Nix is one).

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

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