Korvold, Fae-Cursed King

• WWizards of the Coast2019-10-04
  Official Ruling
  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.
• WWizards of the Coast2019-10-04
  Official Ruling
  If you sacrifice Korvold, its last ability triggers. You won't put a +1/+1 counter on anything, but you'll draw a card.
• WWizards of the Coast2019-10-04
  Official Ruling
  A legendary permanent that is put into a graveyard because of the "legend rule" isn't sacrificed.
• WWizards of the Coast2019-10-04
  Official Ruling
  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.
• WWizards of the Coast2019-10-04
  Official Ruling
  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.

Rulings via Scryfall