The hardest boss within Demoness Ruins was not the final boss, Medusa, but instead the No. 2 boss, Eukana when it was under the influence of Eternal Darkness.
Players who attempted it would need to jump to and fro from various suspended chains within a pitch-dark space. Since they would be unable to see anything, they would need to rely on their judgment and intuition. Due to the absurdity of this mode, only one team had cleared it in the eight years of the game's operation back in Li Yi's past life. When asked about how they felt after clearing it, every single one of the party's members stated that they would rather die than to challenge that boss again.
Being a Hell-level 8-star difficulty boss, Eukana's strength was comparable to the gods. That was how scary the second boss of the dungeon was.