Mandatory Once-per-turn Trigger
When summoned; choose either: (1) target creature currently Prone cannot get up (ever); (2) target creature is knocked Prone; or (3) target Transform creature is considered Transformed and cannot trigger that ability anymore.
Prone: Prone creatures cannot perform actions. They can use an "get up" action to lose their Prone condition.
Transform: When the transform condition is met, you must immediately resolve: "Sac [this]: you can summon from your Extra Deck a [Transformed] creature"

Transformed: Can only be summoned using a [Transform] effect.