Lore:Skull of Corruption
The Skull of Corruption is Vaermina's signature artifact. It is an incredible staff that creates a duplicate, or "clone", of whomever it is cast upon. This clone then attacks the original at the behest of the caster. Legends say that the staff has a mind of its own, and can feed on the memories of those around it.
There is a story about a Thieves Guild Master and the Skull that is probably fiction, but highly amusing. The Master used the Skull on her enemy, creating a clone of him to fight. After defeating the original, the clever duplicate snatched the Skull from the Master and used it on her. Although the cloned enemy could not directly attack the Master, it could use the Skull to create a duplicate Master. The two clones jointly ruled the Thieves Guild for years.
The Skull of Corruption is known to have appeared many times throughout history. In the years leading up to the Warp in the West, an agent of the Blades received the Skull from Vaermina as a reward for killing an "innocent" lich. During the Oblivion Crisis, the Champion of Cyrodiil was also offered the Skull in return for venturing into a tower that manifested the nightmares of a man who stole Vaermina's orb. Vaermina also offered the Skull to the Last Dragonborn if they betrayed and killed a priest of Mara who was conducting a ritual to destroy it. It is unknown if they did as Vaermina requested or allowed the priest to complete his ritual.