Orion

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

1. Primeval Bounty — When Orion enters, Primeval Bounty gains you 3 life
2. Toph, Earthbending Master — When Orion enters, Toph, Earthbending Master puts a counter on a permanent
3. Dancing from Dark to Dawn — When Orion enters, Dancing from Dark to Dawn makes a token
4. Emeria Shepherd — When Orion enters, Emeria Shepherd brings a card back
5. Quest for the Necropolis — When Orion enters, Quest for the Necropolis puts a quest counter on itself
6. Soul Stair Expedition — When Orion enters, Soul Stair Expedition puts a quest counter on itself
7. Nissa of Shadowed Boughs — When Orion enters, Nissa of Shadowed Boughs puts counters on a permanent
8. Thranduil, Sindarin Liege // Silvan Rally — When Orion enters, Thranduil, Sindarin Liege // Silvan Rally makes a token
9. Silvan Reveler — When Orion enters, Silvan Reveler returns itself from the graveyard
10. Floral Evoker — When Orion enters, 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 (Orion is one).

When Orion 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.