Deceiver

"These serpentine demons are Belial's favored servants, and he has granted them some of his skill in cunning and illusion. Deceivers will cloak themselves in the familiar forms of their enemy's companions and friends, waiting to strike when the moment is opportune. It is only then that their true forms are revealed."

- Deckard Cain

Deceivers (a.k.a. Veiled Demons, Doom Vipers, or Snake Demons) are a type of demon

Lore
"The demons came disguised as members of the Imperial Guard. They controlled our minds, drained them to create their illusions."

- Caliem

Deceivers are the favored servants of Belial, who granted them some of his skill in cunning and illusion. Deceivers will cloak themselves in the familiar forms of their enemy's companions and friends, waiting to strike when the moment is opportune. It is only then that their true forms are revealed. Deceivers have been stated to 'drain the minds' of those around them, allowing them to fuel their illusions. They carry two-pronged weapons that can quickly rip a target to pieces.

A number of Deceivers serve Aeshama, guarding her Domain of Horror.

During the End of Days, Deceivers were used in bulk, either replacing members of Caldeum's Imperial Guard, or becoming the Guard itself.

Diablo Immortal
Deceivers are encountered in the Domain of Horror in Diablo Immortal. They can summon Serpent Totems.

Diablo III
Deveivers are encountered in of Diablo III. They are Belial's personal guard, agents and the bulk of his army.

In Adventure Mode, their weaker variants called Abyssals can be encountered.

There are two types of Deceivers in Diablo III:

Deceivers
Like Belial himself, Deceivers are capable of taking any form, disguising themselves as humans. The player first encounters them when they enter the Khasim Outpost, where a bunch of Deceivers led by Maghda are impersonating the Iron Wolves and the Imperial Guard. As soon as their deception is disclosed, they transform back into their original forms, and are to be fought. Later in game, most Deceivers will either attack the player openly or throw their disguise of Imperial Guardsmen (whose ranks, most likely, consist entirely of Belial's demons), or sometimes regular citizens, at the sight of the player.

In addition to that, Deceivers may appear out of nowhere (for scripted events), and their most dangerous skill is invisibility. When they are away from player (not in close combat), they may vanish, becoming invisible (only slight ripple in the air betrays their location) for roughly 3 seconds. In this form, they are impervious to damage, and the first melee hit they deliver will deal double damage, ending the effect. Being ninja-stabbed by multiple Deceivers can prove deadly for an average hero. Fortunately, while they are locked in close combat, they cannot enter invisibility again.

Serpent Magi
Serpent Magi are lacking melee attack (technically they can use it, but will not do so willingly), and may not turn invisible (but may appear out of nowhere or take forms of humans). In return, they create a circular Lightning field around themselves (must stay immobile to channel it) that causes increasing ticks of Lightning damage. Several fields in one area stack. Their goal is to make players flee so that Deceivers may enter invisibility again.

Uniques

 * Goz'turr the Torturer (Wretched Pit)
 * Gryssian (Dahlgur Oasis)
 * Hazzor the Viper (Sewers of Caldeum)
 * Jade Champion (Set Dungeon)
 * Khaaz (Sewers of Caldeum)
 * Khahul the Serpent (Dahlgur Oasis)
 * Lieutenant Vachem (Khasim Outpost)
 * Ningish (Sewers of Caldeum)
 * Ssthrass (Dahlgur Oasis, Ancient Waterway entrance)
 * Tridiun the Impaler (Dahlgur Oasis)
 * Yakara (Eastern Channel)