Korvold, Fae-Cursed King
Legendary Creature — Dragon Noble
Flying
Whenever Korvold enters or attacks, sacrifice another permanent.
Whenever you sacrifice a permanent, put a +1/+1 counter on Korvold and draw a card.
Transformed at his own wedding, he promptly ate the banquet, the gifts, and the guests.
Illustrated by
Throne of Eldraine#329MythicEnglishLegality
Rulings
If you sacrifice Korvold, its last ability triggers. You won't put a +1/+1 counter on anything, but you'll draw a card.
Korvold's last ability is a triggered ability, not an activated ability. It doesn't allow you to sacrifice a permanent whenever you want; rather, you need some other way of sacrificing permanents, such as Korvold's first triggered ability.
If you somehow control no other permanents, you can attack with Korvold. You won't sacrifice anything, and there's no penalty for being unable to do so.
A legendary permanent that is put into a graveyard because of the "legend rule" isn't sacrificed.
If you sacrifice a permanent as part of a spell or activated ability's cost, Korvold's last ability will resolve before that spell or ability.