As of right now, there are 32 cards that have this ability, each marked with a Choose a background text
12, 2026 A World of Treasure and Adventure Await
This means that the Background can have a different color identity from the legendary creature and you use the combined color identity of both the legendary creature card and the legendary Background card to determine what the other 98 cards in your Commander deck (or 58 cards in Commander draft) can be
But for a brief recap: Throne of Eldraine is where we're really firing off the format on all cylinders
The ability only triggers once a turn, but it isnt limited to just your turns, so free spells like Snuff Out and Submerge are perfect to get the most from this cat
Your opponent can respond to the ability or spell afterward, but the creature you sacrificed is already in the graveyard