The Grey Havens

you scry — 420 cards in the game can cause this (The Grey Havens is one).

When The Grey Havens scries:

1. Prudent Fateseer // Peer Review — gives +1/+0
2. Diviner of Victory // Unwind History — gets +1/+1
3. Saheeli, Consul of Oversight — makes a token
4. Celeborn the Wise — makes your creatures bigger
5. Proft, Consulting Detective — puts a +1/+1 counter on itself
6. Matoya, Archon Elder — draws you 1 card
7. Denzilore Fatehold — puts a counter on a creature
8. Elminster — costs less
9. Elrond, Master of Healing — puts a counter on a creature
10. Galadriel of Lothlórien — puts a land onto the battlefield

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

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

When The Grey Havens 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.