Unknown Lord of Darkness
Cloak is a Dark Side ability developed by the Stealth Masters of Mitgarth. This ability, when performed properly, renders the user completely invisible both visually and, to an extent, within the Force as well.
It is known that those particularly strong in the Force can penetrate the Cloak and see the user. There are even instances of such feats being performed subconsciously. During the midnight raid on the White City, Lady Bella, a Stealth Master, could be easily seen by Ganor Vey despite her considerable proficiency with the power. Similarly, Ronan Eiroas was still able to sense her presence, though she remained visually Cloaked. Although when he activated his lightsaber, it focused his senses enough to see her.