Asgardian Citadel

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

When Asgardian 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.

life is gained — 2,076 cards in the game can cause this (Asgardian Citadel is one).

When Asgardian Citadel gains life:

1. Dina, Soul Steeper — makes each opponent lose 1 life
2. Marina Vendrell's Grimoire — draws you that many cards
3. South Wind Avatar — makes each opponent lose 1 life
4. Rodolf Duskbringer — grants indestructible
5. Fiendish Panda — puts a +1/+1 counter on itself
6. Veinwitch Coven — brings a card back
7. Cleric Class — puts a counter on a creature
8. Lich's Mastery — draws you that many cards
9. Wall of Limbs — puts a +1/+1 counter on itself
10. Ratchet, Field Medic // Ratchet, Rescue Racer — triggers

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