There are a number of tropes that are a dime a dozen in RPGs. Let's not get it twisted: tropes aren't necessarily a bad thing; they are just the tools that help to build the outlines of a fantastic story when used correctly. Among all the kidnapped princesses, magical blessings, and chosen ones, we also have the classic epic betrayal.
It can be a sudden and climactic shift in the story that propels the player down a new path. Or it can be a twist that is hinted at and alluded to for hours to build tension. But the change in dynamics brought about by the betrayal can make or break a story's immersion for the player.
10 The Player - Undertale
That's right: sometimes, rather than being the one betrayed, the player is the true perpetrator of the betrayal. When it comes to Undertale, the game tries to guide the player towards the True Pacifist Ending, in which the player doesn't kill a single monster the entire game. But this requires a trip through the Neutral ending route first.
If the player instead goes out of their way to kill every monster in the game, hunting for encounters until nobody comes to battle in each region, the Underground is left barren. Any future playthroughs are forever tainted by the act.
9 Yotsuyu Goe Brutus - Final Fantasy 14
Serving as one of the many antagonists in Final Fantasy 14's Stormblood expansion, she is dethroned from her place as ruler through the main story, and returns in the "The Legend Returns" patch as Tsuyu, a meek and child-like figure.
As the expansion progressed, she began to have memories of her past life, and it seemed as if she would get redeemed. That is, until her adoptive brother showed up, triggering her memories, and tempting her into murdering her parents. After taking the form of a primal, it seems as if the players might be able to save her and bring her to redemption. Sadly, her brother returns and kills her himself. That's two betrayals for the price of one.
8 Ghaleon - Lunar Series
The main antagonist of Lunar: Silver Star Story as well as Lunar 2: Eternal Blue, Ghaleon makes his debut as the Premier of the Magic Guild in the floating city of Vane, and teacher of party member Nash.
When the player reaches Vane, he sends them on a quest to defeat a false Dragonmaster, and even plays the victim when his attempt to kidnap Luna is discovered. Soon after, the players get to use him in battle, fully leveled to 99 and equipped with fantastic gear. So when he is eventually revealed as the antagonist, the Magic Emperor, it comes as a big shock to the party, and one that is hard to forget.
7 Dimentio - Super Paper Mario
Dimentio was initially introduced in Super Paper Mario as one of the subordinates of antagonist Count Bleck. He shows up several times throughout the story to hinder Mario in his quest to stop Black from destroying the world.
In truth, Dimentio was never a minion of Count Bleck but was simply using him as a way to gather the Chaos Heart for himself so that he could remake the world in his image. His betrayal comes right at the very end, attempting to take out Count Bleck, and become the true final boss of the game: Super Dimentio.
6 Sephiroth - Final Fantasy 7
This betrayal is less on the players or the antagonist, but more on the developers for orchestrating it. It is no secret that Sephiroth is the primary antagonist in Final Fantasy 7. The player learns early in just what threat he is, and his presence looms over the party everywhere they go once they leave Midgar.
Aerith is often a staple party member, both for her key role in the story and for fitting in nicely as a healer. So when the game finally reaches its most well-known scene as depicted above, players feel betrayed.
All that hard work they put into leveling her up, unlocking her best weapons and Limit Breaks...Without a single bit of notice, she is gone from the party -- permanently. It's no wonder that many a childhood was spent trying to find some secret way to revive her.
5 Mercer Frey - Elder Scrolls: Skyrim
When it comes to the Thieves Guild and Dark Brotherhood in an Elder Scrolls game, one should always be wary of trusting anybody too much. After all, when you work with thieves or murderers, one should expect to be deceived and potentially killed. But Mercer Frey is a unique case.
Coming from the Thieves Guild in Oblivion where, for the most part, the leadership of the guild was trustworthy, the player is primed to trust Mercer. However, as the storyline progresses, players learn just how duplicitous he truly is, willing to betray and kill anybody that stands in his way, until the player is forced to put the villain down themselves.
4 Riku - Kingdom Hearts
While Riku's actions in the first Kingdom Hearts game start from early on, it does not make them any less of a betrayal. Taking on Riku in Destiny Islands, racing against him, and trying desperately to beat him in his combat training makes you feel like he will be with you to the end. And then the Heartless show up, and Maleficent corrupts his heart, pushing him to act against Sora for the entire journey.
When he shows up once more in Hollow Bastion, he takes away the Keyblade, as well as Donald and Goofy. The player is left feeling powerless with no weapon and no friends by their side. While he does come around, becoming the secondary protagonist in Chain of Memories, that first betrayal is hard for the player to get over (no matter how easily Sora forgives him).
3 Seymour Guado - Final Fantasy 10
Seymour Guado is introduced right off the bat as the villain of Final Fantasy 10. While not overtly stated, his musical motifs, the camera framing, his character design, and even his voice give him a distinctly antagonistic vibe. This isn't helped at all when he chooses to propose to Yuna. But perhaps he's just a bit creepy; after all, he is a major help to the party during his brief playable experience.
Then, the reveal comes that he murdered his own father. He tries to murder the entire party besides Yuna as well as kidnaps Yuna to force her to marry him. Somehow, he also keeps returning to life constantly every time the party manages to kill him, so there's no chance he can be redeemed. His betrayal may not come as a shock, but he does pull it off with quite a bit of flair.
2 Arthas Menethil - Warcraft 3
One of the archetypical betrayals in modern gaming, Arthas Menethis stands out not just for the sheer act of his betrayal, but for the scale of villain he eventually becomes. The primary hero of Warcraft 3's human campaign, the player experiences his descent to madness after he is forced to purge an entire city after it is corrupted with infected grain and the loss of his humanity when he gains Frostmourne and kills his own father.
Now a Death Knight, he eventually returns to Northrend and crowns himself the Lich King, controlling the Scourge that he once fought against. While his rightful place as the Lich King is well known to Warcraft fans nowadays, his rise to the role was a shock to any player of Warcraft 3 when it first launched.
1 Delita Heiral - Final Fantasy Tactics
A friend and close confidant of Final Fantasy Tactics's hero and protagonist Ramza, Delita performs one of the worst betrayals in Final Fantasy history. He spoils both the name and legacy of his friend. Once a close ally to Ramza, he became enraged at the death of his sister by noble hands, an act that Ramza was unable to stop. Delita vowed to no longer be a pawn and to trust nobody ever again.
While he does act to help Ramza a few times throughout the rest of the story, his actions also serve to put Ramza into worse and worse positions. In fact, he eventually forces Ramza to uproot the church and be stripped from the record books then branded as a heretic. Delita ends up as king of Ivalice, no longer a pawn to anybody, and the player, once his best friend, is struck from the annals of history.
Oh, and when the woman he married (the princess Ovelia, now Queen) questions him about all of this, he stabs her. Swell guy!