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The [https://en.wikipedia.org/wiki/Ouroboros Ouroboros] is an ancient Egyptian symbol depicting a snake eating its own tail. It was adopted by the Greeks and most famously used in alchemy. The title may also be in reference to the self-consuming nature of Jack using his powers.

It's shown that consenting to becoming a [[vessel]] does not need to be done verbally. [[Rowena]] gave Michael the needed permission simply by nodding her head. In the past, consent has always been given verbalized by the character agreeing; whether out loudor verbalized subconsciously as when [[Gadreel]] possessed [[Sam]].