Hel

Hel is the Guardian of Eternal Prison.

Before revealing her Ankh, she gives you an opportunity to warp back to the surface and get back all your missing items after solving The Seven Gates of the Underworld if you pick the third option during your conversation with her. The battle should still be fairly simple without needing to do this, though.

Don't let the platforms rise you into the ceiling: You will get crushed and die.

In her panic state, she will try throwing her arm and spear at you. Either get up close if you have enough HP, or spam Flares from below until she's defeated.

After defeating Hel, the Dissonance of Eternal Prison appears. Swallow it with the Beherit.

Hel's third form has been buffed in the 1.1.1.2 update: She slashes faster and occasionally shoots a blue laser from the center of the floor.

Prior to Version 1.5.5.2, the player could get trapped in the Eternal Prison if they didn't ask Hel to teleport them out before fighting them, as all the gates leading out would be closed and the player can't use the Holy Grail.