Unholy Citadel

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

When Unholy 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

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

a creature it grants abilities to — 19,392 cards in the game can cause this (Unholy Citadel is one).

Unholy Citadel gives:

1. Vial Smasher, Gleeful Grenadier — an extra ability
2. Disa the Restless — an extra ability
3. Killian, Decisive Mentor — an extra ability
4. Liliana the Repentant — an extra ability
5. Massacre Girl, Most Wanted — an extra ability
6. Elas il-Kor, Sadistic Pilgrim — an extra ability
7. Rankle and Torbran — an extra ability
8. Caesar, Legion's Emperor — an extra ability
9. Far Fortune, End Boss — an extra ability
10. Ayara, First of Locthwain — an extra ability

19,382 other cards care about it too. These are the ones with the most connections.