Endless Impostors

In Neil Gaiman's critically acclaimed The Sandman, among the most important characters are the Endless. The Endless are cosmic beings that embodied the concept they named after (Dream is the lord of dreams, Death is a Grim Reaper-like pyschopomp and so on). In some of the spin offs, there are several people that claim are Endless, but are lying.
Dread

Echo and her army

Echo was introduced as the transvestite lover of a person attacked by the original Corinthian (a living nightmare). Echo and Gabriel Ashe became serial killers. The second Corinthian and man-turned-bird Matthew stopped them resulting in Gabriel dying and Echo losing an eye. Her life went downhill. Thieves knocked her out and found her "dead".  In reality, she was taken to the Dreaming (the domain of Dream) and reborn as the third Corinthian (as the second one was turned mortal). She joined Brute and Glob's attempt to rebel against Dream. They went to the House of Mystery and Secrets. The Houses gave her massive powers (by having her absorb the Nightmare army). They also brainwashed her to think she was an Endless member named "Dread" and that Dream imprisoned in the souls of men, but she got free. Cain managed to de-power Echo by saying out the truth (thus it was no longer a mystery or secret). Dream told her that she would be unmade, but he thought he would sooner or later find a place her in his realm.
Duplicity and Deceit

Garamas and Gyges

The titans Gyges and Garamas learned the Presence (the divine creator of DC Universe) abandoned creation. They were worried since their positions of authority were now in danger. The duo decided to break into the Dreaming and take control of it. Upon arriving, they are greeted by Lucien (a servant for Dream). They claimed to be Duplicity and Deceit of the Endless and were here for family business. Lucien saw through their deception and pointed out duplicity and deceit are synonyms for each other while the Endless are supposed have different domain. Turned away, they decided to become the Presence themselves.
References:
http://www.comicvine.com/the-endless/4060-40483/
The Sandman volume 2: "The Doll House" and volume 9: "The Kindly Ones"
http://en.wikipedia.org/wiki/Endless_(comics)
http://www.comicvine.com/the-presence/4005-56244/