When the rune warrior passed through the Clockwork Crypts, he found the hidden rooms that were guarded by these creatures. The rooms contained lots of chests which need elemental coins to unlock. He then had the choice to evade them carefully or to risk the fight with them.
The demons do have an insane amount of health and regenerate their health quickly. Also they possess an immunity against all kinds of magic. Everytime they attack, they have a chance to inflict a low-level fireburst effect on their targets. The demon is not supposed to be killed, but if you have a black mage with necromancy 20 and the blades spell, it is possible, but it will take a long time!