도적아키타입에 어쌔신 있잖아요

Assassinate

Starting at 3rd level, you are at your deadliest when you get the drop on your enemies. You have Advantage on attack rolls against any creature that hasn’t taken a turn in the combat yet. In addition, any hit you score against a creature that is surprised is a critical hit.

이런 능력을 가지고 있던데 은신하고나서 공격하면 서프라이즈 판정을 받는지 궁금하고, 우선권이 느린적에게 이점을 받는다고 되어있는데 우선권은 한 번만 굴리는지도 궁금합니다(혹은 매 턴마다 굴리는건지)