Umbriel

Umbriel is the floating city, ripped straight from the realm from where the Hist originated. The shadow it casts causes death to those in it, the corpses are then reanimated and turned into the Undead. It keeps afloat thanks to the Ingenium, which kept afloat part of Vivec city. It's theft can be credited to the great eruption of Red Mountain, which destroyed most of Morrowind. Umbriel first appeared in Tamriel 40 years after the Oblivion Crisis. It is powered by the soul of Umbra, who's escape has greatly weakend Clavicus Vile.