Alarum

a creature untaps — 519 cards in the game can cause this (Alarum is one).

1. Fishing Pole — When a creature untaps thanks to Alarum, Fishing Pole triggers

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

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

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

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