Tarnished Citadel

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

When Tarnished Citadel 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 deals noncombat damage — 3,068 cards in the game can cause this (Tarnished Citadel is one).

When Tarnished Citadel deals noncombat damage:

1. Quest for Pure Flame — puts a quest counter on itself
2. Fear of Burning Alive — deals that amount damage
3. Niv-Mizzet, Visionary — draws you that many cards
4. Chandra's Incinerator — deals that much damage
5. Chandra's Pyreling — gets +1/+0
6. Talon of Pain — puts a charge counter on itself
7. Asgardian Inspiration — returns itself from the graveyard
8. Thor, Guardian of Midgard — exiles a card from your library
9. Virtue of Courage // Embereth Blaze — exiles a card from your library

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

life is lost — 3,160 cards in the game can cause this (Tarnished Citadel is one).

When Tarnished Citadel makes a player lose life:

1. The Master of Lake-town — mills a card
2. Intermediate Chirography — puts a counter on a creature
3. Vilis, Broker of Blood — draws you that many cards
4. Vengeful Warchief — puts a +1/+1 counter on itself
5. Wax-Wane Witness — gets +1/+0
6. Oath of Lim-Dûl — sacrifices an enchantment
7. Marina Vendrell's Grimoire — triggers
8. Lich's Mastery — triggers
9. Lich's Tomb — sacrifices a permanent
10. Gonti's Machinations — puts a counter on a permanent you control

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

anything is dealt damage — 19,812 cards in the game can cause this (Tarnished Citadel is one).

When Tarnished Citadel is dealt damage:

1. Darien, King of Kjeldor — makes that many tokens
2. Blood Hound — puts that many +1/+1 counters on itself
3. Angelheart Vial — puts that many charge counters on itself
4. Sun Droplet — puts that many charge counters on itself
5. Living Artifact — puts that many counters on a permanent
6. Swarmborn Giant — sacrifices itself
7. Risona, Asari Commander — triggers
8. Azor's Elocutors — triggers
9. Lich — sacrifices a permanent
10. Contested Game Ball — untaps itself

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