Favourite eldritch abomination?

[Alardem]

The eponymous house from the pretentious novel House Of Leaves. The sheer bizareness of the endless staircase, the invisible demon lurking within, its ability to change dimensions whenever the author deems it scariest - it can make Silent Hill seem pleasant.

For the internet kids, it's also likely the inspiration for SCP 087 AKA the endless staircase.