Darksteel Citadel

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

When Darksteel 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.

a nontoken artifact enters the battlefield — 3,768 cards in the game can cause this (Darksteel Citadel is one).

When Darksteel Citadel enters:

1. Ultron, Artificial Malevolence — makes a token
2. The Prydwen, Steel Flagship — makes a token
3. Conquest Mission — puts a quest counter on itself
4. Surge Conductor — proliferates
5. Replication Specialist — makes a token
6. Weapons Manufacturing — makes a token
7. Mirrorworks — makes a token

an artifact enters the battlefield — 4,643 cards in the game can cause this (Darksteel Citadel is one).

When Darksteel Citadel enters:

1. Transplant Theorist — draws you 1 card
2. Elvish Archivist — puts 2 +1/+1 counters on itself
3. Golbez, Crystal Collector — surveils
4. Sludge Strider — makes a player lose 1 life
5. Underhanded Designs — makes each opponent lose 1 life
6. Mechan Assembler — makes a token
7. Glassdust Hulk — gets +1/+1
8. Rook Turret — draws you 1 card
9. Hedron Detonator — deals 1 damage
10. Pactdoll Terror — makes each opponent lose 1 life

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

anything enters the battlefield — 26,499 cards in the game can cause this (Darksteel Citadel is one).

When Darksteel Citadel enters:

1. HYDRA Assault Robot — deals 1 damage
2. Era of Innovation — puts a counter on a permanent you control

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