Phlegethon

A river of blood in Hell that burns human souls...also a woman's period.

She had Phlegethon in her pants.

To venture into the Phlegethon

Expression refering to the willingness of an individual to perform felation on their sexual partner regardless of them being on their period. The Phlegethon being a river of boiling blood where violent souls are punished.

We hadn't seen each other in a while, I did not care I had to venture into the Phlegethon.