The set of creatures affected by Paladin Danse's last ability is determined as the ability resolves
For example, since flipping Sephiroth only requires four Creatures to die, regardless of who controls them, some of the strongest cards in the deck are edict Creatures, or Creatures that force each player to sacrifice when they enter
It's a key part of keeping players with the game long term
Lets say I have the following lessons on my sideboard: Environmental Sciences, Containment Breach, and Mascot Exhibition
It also gives a reason to attack me, and that's anti-syngery, but it could also be good recurring draw + punishment for trying to take it from me, and the deck is likely good enough at taking the crown back if necessary