When creators want to emphasize the similarity of traits between the forces of the divine and the forces of the infernal, several strategies are used. This trope is about the strategy of similar appearance. Mortal characters encountering both the divine person and the infernal person must rely on subtle clues instead of obvious markers.
In many works of fiction, angels and demons are depicted with obvious markers. Like the two are distinct "races", and often polar opposites. Divine creatures (including good gods) are often Winged Humanoids that glow with Angelic Beauty, while Infernal creatures (and evil gods) are Big Red Devils with horns, cloven feet, and nasty big pointy teeth. You'd never mistake one for the other, or even think they have much in common. But this trope is for when you can confuse them. When the two are very similar, and easily mistaken for each other.
If there are any distinctions in appearance at all, they are likely to be subtle. Perhaps Good Wings, Evil Wings are involved, with the two groups looking otherwise identical. Or differences in costume while Acting for Two, where the same actor is used to portray both a Christian God/angel and Satan/demon. That specific technique is most likely codified by George Burns in Oh, God! You Devil. The similarity of appearance is Older Than Feudalism: The story of the Old Testament is that Lucifer (Satan) himself was the most powerful of the Heavenly Host, before deciding to turn away from his God. He can still appear as an angel of light because he is one, not because he's disguised. In other words, the difference between angels and demons is the same as between a good human and an evil one.
This trope can be used to complement other similarities. The creator may be trying to show the two groups are Not So Different or God Is Evil. It can also be used to explain why a normally good person is willing to make a Deal with the Devil, instead of turning it down out of principle.
Beware Beauty Equals Goodness: just because an infernal creature can choose to have Angelic Beauty doesn't mean they are angelic. Compare Light Is Not Good, just because a character looks good, doesn't mean they actually are. Compare Face of an Angel, Mind of a Demon, where a person with an evil mind has a beautiful and innocent face. Compare Fallen Angel, where an angel becomes an advocate of evil, and Ascended Demon, where a demon becomes an advocate of good. See also Our Demons Are Different and Our Angels Are Different.
- Devilman features the character of Satan as a glowingly beautiful hermaphrodite with twelve golden wings.
- Digimon Frontier has an expy of Devilman's Satan in Lucemon, who is also a beautiful childlike angel with twelve white wings. His second form is a bishonen, handsome man with half his wings now stained black, resembling bat wings. His highest forms abandon this in favor of a monstrous dragon that hides a divine looking gigantic fetus-like worm that fits in some respects with golden wings.
- Lucifer. In this series (a spinoff of The Sandman), Lucifer is depicted as a handsome man with golden wings.
- An Inverted Trope in the New-52 version of Resurrection Man, where the angels that Mitch runs into sometimes look (and act) a hell of a lot like demons.
- The Sandman has Lucifer, Be'elzebub, and Azazel, the Demon Lords and Archdevils of Hell. Each of them are depicted with a different appearance, making this trope played straight and averted. Lucifer looks like his angelic biblical appearance, while Azazel appears made of shadows, eyes, and teeth, and Be'elzebub is a giant bug.
- Hellblazer occasionally has demons looking like angels to trick mortals.
- In Cosmic Christmas, Satan assumes his angelic (Lucifer) form in an attempt to fool Archangel Gabriel into thinking he never rebelled and the whole uprising was just a test by him and God to test Gabriel's faith. It's implied only he can do this, as a previous demon Gabriel encounters who pretends to be trying to return to the angels remains in a demonic form.
- The Dresden Files varies this. On the one hand, it has Chaunzaggaroth a.k.a. 'Chauncy', an oversized monstrous, chitinous looking monster with an Oxford accent, spectacles, and an Affably Evil demeanour, and most of the Denarians, Fallen Angels with mortal hosts who usually transform said hosts permanently (or just in combat) into monstrous looking creatures. On the other, it has other Denarians, like Lasciel - or rather, her shadow in Harry's mind, 'Lash' - who routinely appears to Harry in the guise of an attractive young woman, sometimes even more or less literally angelic. She even explicitly lampshades this, pointing out that she used to be a resident of Heaven and can still put on the look - before somewhat drily asking if she should add the wings and the harp.
- Good Omens. The narration notes that there's "not that much difference" in appearance between angels and demons, except the demons are better groomed. One of the running themes of the book is that angels and demons are Not So Different.
- Lucifer-Satan in Left Behind appears in many guises, but his true form is a wonderful angel of light. His appearance is so awe-inspiringly beautiful that even a saintly old Christian scholar who has spent his whole life studying the Bible immediately wants to worship him when he sees him in a vision, and has to be brought back to his senses by a good angel.
- In Paradise Lost, Satan is able to disguise himself as a cherub to fool Uriel into allowing him to travel to Earth.
- In Robert Louis Stevenson's short story "Markheim" the protagonist meets a supernatural creature and assumes it is the devil. The creature offers to help him with the crime Markheim is committing -apparently a classic temptation from the devil. Markheim resists the temptation and calmly admits his crime and waits for the police, knowing full well he will hang for it. Unseen to Markheim the creature undergoes a strange transformation and is triumphantly happy with Markheim's decision. The ending suggests it might actually have been the exact opposite.
- The Silmarillion: Sauron's "angelic" guise as Annatar, Lord of Gifts, belied his demonic nature and enabled him to manipulate the Elves and Númenoreans. However, he ends up with Shapeshifter Mode Lock due to being caught in the destruction of Númenor, meaning he is unable to return to his fair form after that.
- The demons in the Book of Swords make themselves appear this way to Vilkata, as they provide his vision. He isn't fooled for a moment, but given his character, it doesn't stop him from making use of their services.
- Occurs in surprising crossover with Face of an Angel, Mind of a Demon in My Best Friends Exorcism. While Gretchen is still trying to fight off the demon from Hell that is trying to possess her, everyone is horrified by how ugly she is (as she refuses to shower or sleep). The minute she gives in, Gretchen regains her perfect "angelic" appearance and then some.
- In The Monk, when Satan is first summoned by Matilda, he appears as an Angelic Beauty whose only demonic attributes are the black color of his wings and a subtle moodiness to his expression, which is probably done to trick Ambrosio into thinking that he's easy to bargain with. When Ambrosio gives into temptation at the end of the novel and summons him to make a Deal with the Devil, he appears as a more traditional ugly horned devil instead.
- The series Brimstone follows Detective Ezekiel Stone, as he is sent on a mission to hunt down 113 damned souls that escaped from Hell. Satan visits at least Once an Episode, played by John Glover and wearing a good-looking business suit. When an angel appears later on, John Glover is dressed in a very worn-out shirt and pants outfit. Ezekiel first mistakes him for Satan, wondering why the new outfit.
- In the Star Trek: Deep Space Nine episode "Penumbra" the Pah-Wraiths (Prophets exiled from their home in the Wormhole) appear to Kai Winn Adami in a vision, claiming to be the Prophets.
- An example of Acting for Two was found in an episode of Touched by an Angel, where an evil twin of Monica (one of the regular angels "touching" human lives) showed up.
- In Supernatural, both angels and demons require host bodies to manifest, making their appearance similar in that sense. But higher tier demons are also capable of calling upon holy white light (like angels!), one of the most powerful spells of the series.
- Kamelot. In the song "Descent of the Archangel", Mephisto (named for a demon in The Bible) appears to Ariel in his full angelic glory, offering Ariel a bargain for his soul. Lampshaded by the demon:
"Speak, my friend, you look suprised —
I thought you knew I'd come disguised
On angel wings, in white..."
- As pictured on the trope page, Lucifer in the Shin Megami Tensei franchise can appear as his original self: the seraphic "Helel". Whether it's his true form or not is really dependent on the game: sometimes it is, sometimes it's a disguise, sometimes Helel is treated as a separate demon entirely, or not even a demon at all.
- One of the most basic powers that a Fallen Angel has in Nexus Clash is the ability to appear as a good angel. This lasts until someone notices the trail of bodies...or tries to heal them, since this is a setting where No Cure for Evil is very much in effect.
- In The Elder Scrolls, this is both played straight and subverted in different instances for the Daedric Princes, who are (very loosely) the "devils" to the Aedric "angels". The Princes are technically divine beings Above Good and Evil who operate on their own scales of Blue-and-Orange Morality depending on the spheres over which they govern. They can take any form they choose, from "divine" to outright Eldritch Abominations, though most stick to a humanoid form when dealing with mortals. The best straight example is Meridia, who is associated with Life Energy, Light, and Beauty. She has a Fallen Angel backstory and is typically depicted as a beautiful woman, sometimes with angel-like wings. She is typically considered one of the "good" Daedra, though has some very Good Is Not Nice moments, is a fan of Disproportionate Retribution to those who anger her, and hates anything undead with such a passion that no cost is too great to wipe them out.
- The Devil May Cry series quite enjoy this trope:
- The original game's Big Bad Mundus initially appears in a massive, cathedral-like chamber, his stone visage carved to resemble a handsome, three-eyed angel. As the battle rages on and his statuesque form is ripped apart, his true grotesque demonic features are revealed underneath.
- The third game features late-game enemies known as "The Fallen." They appear to be winged angels, but enough attacking destroys their protective wings and reveals hideous demonic faces.
- A number of members of the Corrupt Church the Order of the Sword are the results of experiments to combine human and demon DNA. Despite this, their demonic forms more strongly resemble angels.
- It isn't simply the demons that are affected by this trope- certain areas of Hell seen in the first and third game resemble distorted versions of Heaven and the locations are filled with very deceptive light.
- Pride from Darksiders III is one of the Seven Deadly Sins, a group of demons so powerful and dangerous that the Charred Council had them imprisoned shortly after God created them to protect the rest of the universe from their influence. With that said, her design from her almost-human physiology, her blue angelic wings, and her purple and gold armor would have you believe her to be an angel. The only giveaways would be her black cape, her broken-looking halo and her mangled face underneath her mask.
- Hell in SaGa Frontier looks like Fluffy Cloud Heaven, and its demons in turn look like winged cherubs. Until you touch one and the fight starts, at which point the illusion drops.