Mana Confluence

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

When Mana Confluence 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 lost — 3,160 cards in the game can cause this (Mana Confluence is one).

When Mana Confluence makes a player lose life:

1. The Master of Lake-town — mills a card
2. Intermediate Chirography — puts a counter on a creature
3. Vilis, Broker of Blood — draws you that many cards
4. Vengeful Warchief — puts a +1/+1 counter on itself
5. Wax-Wane Witness — gets +1/+0
6. Oath of Lim-Dûl — sacrifices an enchantment
7. Marina Vendrell's Grimoire — triggers
8. Lich's Mastery — triggers
9. Lich's Tomb — sacrifices a permanent
10. Gonti's Machinations — puts a counter on a permanent you control

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