Professor Onyx

For Professor Onyx's second loyalty ability, if an opponent has multiple creatures with the greatest power, that player choose which one to sacrifice. The next opponent in turn order chooses which creature they are sacrificing, then each other opponent in turn order does the same, knowing the choices made before them. Then all chosen creatures are sacrificed at the same time.

If Professor Onyx seems familiar, it's probably because you've seen this type of planeswalker before.

Each magecraft ability has a different effect, although they all have the same trigger condition, whenever you cast or copy an instant or sorcery spell.

For example, if you control Archmage Emeritus and cast an instant or sorcery spell, Archmage Emeritus's magecraft ability will trigger and you will draw a card.

If an effect creates multiple copies of an instant or sorcery spell, magecraft abilities trigger once for each copy created by the effect.