The Machine

One of the Two bosses of Floor 6.

Lore

It's implied to be one of the prior visitors. As shown by it's ship like design in the final phase of the battle. Which is very similar to how Monolith appears after Null turns into that form.

It's a construct made by the Council, infused with the Power Eternal. Due to their "Impending Mortality". Which implies it was made because they knew that a disaster was about to occur. It had the side effect of opening the "Eternal Domain".