ChampionChampion represents two triggered abilities. “Champion an [object]” means “When this permanent enters, sacrifice it unless you exile another [object] you control” and “When this permanent leaves the battlefield, return the exiled card to the battlefield under its owner’s control.”
The two abilities represented by champion are linked. See rule 607, “Linked Abilities.”
A permanent is “championed” by another permanent if the latter exiles the former as the direct result of a champion ability. (CR 702.72) a Merfolk (When this enters,
sacrificeTo sacrifice a permanent, its controller moves it from the battlefield directly to its owner’s graveyard. A player can’t sacrifice something that isn’t a permanent, or something that’s a permanent they don’t control. Sacrificing a permanent doesn’t destroy it, so regeneration or other effects that replace destruction can’t affect this action. (CR 701.21) it unless you
exileTo exile an object, move it to the exile zone from wherever it is. See rule 406, “Exile.” (CR 701.13) another Merfolk you control. When this leaves the battlefield, that card returns to the battlefield.)
Whenever this creature deals combat damage to a player, you may
sacrificeTo sacrifice a permanent, its controller moves it from the battlefield directly to its owner’s graveyard. A player can’t sacrifice something that isn’t a permanent, or something that’s a permanent they don’t control. Sacrificing a permanent doesn’t destroy it, so regeneration or other effects that replace destruction can’t affect this action. (CR 701.21) a Merfolk. If you do, take an extra turn after this one.