Blue Hydra

Blue Hydra is a demon found in Iris and the Giant.

"The hydra! The heads of the two first rows can attack. My thunder will target only one."

Location
The Blue Hydra can appear as both a boss and not. The non-boss variant does not count towards the Game Book.

The boss variant can be found on Floor 12 in Path of the Giant as a group of 9, and Floor 14B in Path of the Ferryman as a group of 3 among other enemies.

The non-boss variant can be found on Floors 2, 3, 4, 20, 23 & 24 in Path of the Ferryman, and additionally Floor 14 on Nightmare difficulty.

Strategy
The Blue Hydra can attack you from the first or second row. When attacked, it will appear decapitated and be unable to attack you. It must be attacked again to be defeated, however if your turn ends it can regenerate. It takes 2 turns for the Blue Hydra to fully regenerate (or 4 on Easy difficulty), any attack before then will kill it regardless of the stage of regeneration.

The Thunder card's multiple enemy targeting will not work on the Blue Hydra. Having multiple Axe or Fire cards is recommended for taking them down quickly.