Geas

Also spelled alternately as geis, a geas is a form of magical compulsion or curse that originates in Celtic mythology. Those under a geas are required to fulfill certain conditions or are otherwise changed. One of the most famous is that of Cuchulainn, who was under numerous geasi such as that he must never eat the meat of a dog or refuse food offered by a woman. When an old hag offered him dog meat, he was forced to break one geas or the other, which eventually led to his death.

A geas usually takes the form of either a command or a prohibition: "You shall do this," or "You shall not do this." In practical terms, the geas may be prophetic, bringing about its own fulfillment either through manipulation of cosmic events or by simply instilling into the subject a compulsion which he cannot resist. If the geas can be broken, doing so will bring about the death of the subject, either directly or by cosmic retribution.

Compare: Magically-Binding Contract (which can have similar effects but is more of a deal rather than a spell or curse), Restraining Bolt (where an object produces similar effects)

Anime

 * In Code Geass (the title of which this trope inspired), Lelouch has the ability to place a person he makes eye contact with under a geas, as they have to do whatever he tells them to do once Lelouch's "Geass" power kicks in (though it only works once per person). The "Geass" powers in this series often work more like a psychic power or ability (such as making people fall in love or mind reading), which makes the majority of those powers unrelated to this trope. Only Lelouch appears to have the ability to lay a proper geas on people.
 * The Expanded Universe introduces more characters with the same type of power as Lelouch. Rai, protagonist of the Visual Novel Lost Colors, has a version based off of hearing rather than eye contact, while the Nintendo DS RPG's Big Bads Castor and Pollux have "The Bless", which works on the same person more than once.
 * A very interesting case in Bleach is Giriko and his Time Tells No Lies ability. Essentially, it's a contract he envokes on himself or on others, with the spirit of his watch. It can take multiple forms; increased strength, the power to kill by line of sight, and so on. The catch is that no party involved can violate this contract, or they'll be incinerated--the part that makes it a Geas. Wonder what ever happened to Sadly, Giriko doesn't see much action.
 * In more of a direct mind control example, Zommari the 7th Espada can hit you with a spell from any one of his 50-something eyes, controlling whatever he hits. If he hits your head, he gets your whole body.

Comic Books

 * Dunstan in A Distant Soil is a Fair Folk, and he says he is under a Geas that he can't tell a mortal soul who he really is while he's on mortal soil. However, it applies more when he's on soil - when he's on a cruise ship or a spaceship, he can actually mention this without violating it.

Literature
"Rob: 'Tis a heavy thing, tae be under a geas. Daft Wullie: Well, they're big birds."
 * The otherworld beings from the Bartimaeus books are automatically compelled to obey their orders from magicians by something perhaps like an obsessive compulsive disorder.
 * Bartimaeus explain that obeying the orders is necessary since there was one djinni that refused to kill another djinni it loved. The djinni's repeated refusal of order tore his essence and caused him to explode and destroying the prince commanding him and the prince's entire palace. Since then magicians tend to be more careful about which djinni they summon.
 * It's also in the djinn's best interest to just do what the magician commands and get it over with, lest the magician cast the Curse of Indefinite Confinement or the Curse of the Shriveling Fire, which is exactly as unpleasant, painful, and deadly as it sounds.
 * Features into the backstory of Diarmuid in Fate/Zero. He was bound to serve his lord, but also had a curse on him that caused women to fall instantly in love with him, which is worse than it sounds. The lord's fiancee succumbed to it and placed him under a geas to make him run away with her. They eventually got married with the consent of said lord, but it ends up getting Diarmuid killed and he wants to repent.
 * Not stated to be such, but this is more or less the case with the Witch King of Angmar in The Lord of the Rings. It was prophecied that he shall not fall by the hand of Man, and in the end he is taken down by a hobbit and a woman.
 * Used in The Laundry Series by Charles Stross, which is essentially MI 6 meets H.P. Lovecraft. Figures prominently into the second book, The Jennifer Morgue where the protagonist is put under a reality-warping geas that essentially transforms him into a James Bond-esque hero (Turns out ).
 * Jo Walton's Tanagiri novels feature several geases, courtesy of the Irish Fantasy Counterpart Culture.
 * In the The Witches Of Eileanan series several people are put under Geasa. Most of them are, or given them by, Khan'cobans (Like Inuit elves with ram's horns), though there isn't a spell involved, it's mostly just a task or obligation that is given social significance if one were to break it or accomplish it. It's less a spell and more a binding of honor.
 * In the Cassandra Palmer series, the Geis is like a love spell, a magical claim that warns off any would-be suitors and compels the the two people to be attracted to each other.
 * In Deep Secret, Rupert defeats one of the villains by laying a geas on him such that if he tries to use magic again he'll die.
 * Discworld:
 * Parodied in Sourcery, where everyone else thinks the guy under the geas is talking about geese, leading to much confusion.
 * Parodied again in A Hat Full of Sky and Wintersmith, where Rob Anybody is put under a geas by his wife, Jeannie, to protect Tiffany Aching. It becomes a Running Gag that Daft Wullie keeps thinking Rob means an actual goose.


 * In Clark Ashton Smith's "The Seven Geases", the protagonist is put under of a sequence of, well, seven geases.
 * In the Lord Darcy story "A Case of Identity", Lord Seiger is by nature a conscienceless psychopath; a very extensive geas has been laid upon him never to hurt anyone save at the direct order of his superior in the King's Service. He shows no signs of resenting this, but he clearly enjoys those moments when he's let off the leash.
 * Fairies in Artemis Fowl are under a geas set by their first king, Frond, to never enter a human's dwelling without permission. The idea was that fairies were mischievous and would abuse the humans. Over time, the magic has faded a little, but is still binding.

Mythology

 * This is over the place in Celtic Mythology, from which it originates. Diarmuid, Cuchulainn, Lleu Llaw Gyffes and more.
 * Several heroes, including Cuchulainn and high king Conaire Mór, found themselves in situations where they were forced to choose between either breaking their geas or violating the customs of Sacred Hospitality. They chose the former, leading to their inevitable death. Normally in the form of a particularly heroic Last Stand.
 * In Cuchulainn's case it wasn't even a matter of Honor Before Reason; he was under conflicting geasi, which his enemies exploited.

Tabletop Games

 * There is a spell called geas in Dungeons and Dragons that forces the character to fulfill the terms. Clerics call the same spell "Quest", and also have a "Mark of Justice" that places a curse on a character who breaks the conditions of the Mark.
 * The powers of Wu Jen and characters under the various Vows of the Book of Exalted Deeds have specific behaviors or tasks they must perform to maintain their powers.
 * Geasa were used in 3rd edition Shadowrun to recover points of magic that had been lost by shaman/magicians/adepts. Your character had to accept some sort of condition to recover a point of magic. Usually anything the GM wanted/was willing to allow but classic ones were some sort of talisman that you had to keep on your person, having to fast on a regular basis, spend time in meditation, or only use magic in certain circumstances. Breaking the geas reduced your magic back to it's normal level.

Video Games

 * Lancer in Fate/stay night, whose true identity is, has a geas that if a man from Ulster uses Caladbolg against him, he must lose the fight, . The sequel, Fate/hollow ataraxia has Shirou play a trick on him involving his original two geas, though they are never stated outright: Three female friends of his from school offer Lancer a hot dog, an offer he can't refuse and (were it actually dog meat) something that could potentially kill him. This is actually related to how he originally died.
 * At one point in Unlimited Blade Works Rin threatens to use Geas on Shirou.
 * The Halo novel Cryptum has a technological variant that can be imposed by Forerunners on other species (the term geas being the closest word in human vocabulary to describe the condition). The Librarian imposed one on the entire human race to make sure her husband was found and awakened at the proper time, the compulsion being that the humans present at his location would unknowingly sing a song that contained the codes needed to allow his reviver passage. Some Forerunners believe that their forerunners, the Precursors, had imposed a geas on Forerunners as well.
 * Primordium takes it farther. One character has to deal with rejecting a geas that forces her to go to the dreaded Palace of Pain, which would ensure her and her companions' deaths. Turns out, gei are subject to change via nearby beacons, and the ones on that particular Halo have been hijacked by the less favorable side in the Enemy Civil War.
 * Another aspect of the human geas is that the memories, and eventually entire personalities, of ancient humans are carried by many humans, "germinating and blossoming" as they do/see/hear/etc. something that triggers it. It gets to the point where the "old spirits" can hijack their host's body to communicate. None of this is considered a pleasent experience, to say the least.
 * In Neverwinter Nights: Hordes of the Underdark, as a way of avoiding the But Thou Must! faux-choice, the player character gets a geas to kill the evil sorceress Valsharess.
 * in Baldurs Gate II is under a geas to betray you at a certain point of the plot.

Web Comics

 * In Order of the Stick, Belkar is put under a spell which prevents him from dealing lethal damage (a D&D rules term, basically meaning any damage that isn't the kind you'd use in a sport fight -- and Belkar never bothers with sublethal damage when he can get away with lethal) to any living thing within the bounds of a settlement. He also cannot travel more than a mile from Roy, on pain of suffering from a sickening curse. The curse is eventually.

Web Original

 * In The Gamers Alliance, the archdemon Malphas ends up under a geas when he kisses fellow archdemon Nina Heeate's enchanted ring, and the geas forces him to serve Nina's every whim. It turns out that Nina was using magic and her female physique to mess with Malphas's mind, which confused Malphas enough to make the geas take effect. Nina doesn't keep Malphas on a tight leash, however, and instead lets him do what he wishes as long as he doesn't stand in her horde's way. The geas also requires Malphas to keep his mouth shut about the whole endeavour, further ensuring that Nina's part in the whole mess stays out of the limelight.

Western Animation

 * An unusual example in Gargoyles; Demona once placed Goliath under a spell compelling him to obey the spoken orders of whoever held the spellbook that was the source of the spell. In a rare bit of Genre Savvy, she then tore out the pages containing the spells and destroyed them, so that no one could undo the spell. After the rest of the Gargoyles defeated Demona, Eliza found a way around it; holding the spellbook, she commanded Goliath never to behave in a way that went against his nature. So essentially, for the rest of the show's run, Goliath was under a geas--to be himself.