The world of Elden Ring is full of subplots and sidequests, usually given to you by completely missable NPCs who travel around the Lands Between, stopping at various landmarks along the way. Some of them are the keys to new weapons, armor, or spells, while others will unravel whole layers of lore and in-game mythology that will lead you to alternate endings and, maybe, change your understanding of your mission as a would-be Elden Lord altogether.
This is certainly the case with the Frenzied Flame, one of Elden Ring's most dense (and most easily missed) side stories that will walk you down an entirely new path on your quest to the Erdtree--which will even make for some new enemies and some ominous foreshadowing of the future, should you choose to see this quest through.
So what, exactly, is the Frenzied Flame and what does it have to do with Elden Ring's story? The answer is a little complicated, so strap in--and brace for a lot (and we do mean a lot) of major game spoilers along the way.
The World, As It Is
One of the key components of Elden Ring's internal mythology is the idea of The Greater Will, which is what's known as an "Outer God," or an eldritch force that is exerting its influence on the Lands Between via conduits--regular gods, like Marika. There's a lot of finer points and ins-and-outs to being a god, outer or otherwise, in Elden Ring, but for now, its best to understand the Outer Gods in this story as puppet masters trying to pull the strings of the Lands Between while also existing firmly outside of the normal concept of reality.
The Greater Will uses Marika as a conduit, but it also uses monstrous hand-like creatures known as the Two Fingers to communicate. At some point in the past, it established the idea of The Golden Order, a sort of quasi-religious movement anchored by the communication of the Two Fingers and Marika's reign, that believed in a divine plan or set of rules for all life. In fact, we even have the names of two of those rules in-game, as incantations known as the Law of Causality and the Law of Regression.
Put simply, The Greater Will has a very strict view of how things work and how life must function--and, unsurprisingly, not everyone is super eager to go along with it, especially if they don't quite see eye-to-eye with those rules. One such dissenter is another Outer God, popularly known as The Frenzied Flame.
The Frenzied Flame is, in essence, the exact opposite of The Greater Will. Where The Greater Will instills rigid order and strict laws, The Frenzied Flame instills--well, frenzy. Absolute chaos, madness, and in the words of some of their faithful, a complete breakdown of everything that "divides and distinguishes."
The Greater Will may have been the top dog in terms of influence in the Lands Between prior to the Shattering and subsequent decay, but it was never without challengers. It's hard to tell just when or how The Frenzied Flame began to take root within the population but evidence suggests it's been around for a while--and, in fact, has been actively buried by The Greater Will's loyalists in an effort to keep it hidden and locked away. There's only one piece of concrete text in the game pointing to any sort of origin, found on the incarnation the Howl of Shabriri which reads: "it is said that the sickness of the flame of frenzy began with Shabriri, the most relieved man in all history."
More on Shabriri in a second.
Now, while these two are far from the only Outer Gods in Elden Ring's mythology, they seem to be the two most closely related to one another. While The Greater Will uses the Two Fingers as conduits, The Frenzied Flame uses a creature called the Three Fingers for itself. This could potentially suggest that the two of them are, in fact, one entity that was bifurcated somehow into two opposing forces.
This is further implied by dialogue later in the game with an NPC, Hyetta, who explains: "All that there is came from the One Great. Then came fractures, and births, and souls. But the Greater Will made a mistake. Torment, despair, affliction... every sin, every curse. Every one, born of the mistake. And so, what was borrowed must be returned. Melt it all away, with the yellow chaos flame. Until all is One again." Though, the possibility for unreliable narration certainly shouldn't be discounted.
So, now that the Shattering has occurred, many of the Fingers are dead, and the Lands Between is in a state of extreme decline, The Frenzied Flame is among the many powerful forces trying to make a move. To do this, it, like the other Outer Gods, needs the help of some powerful loyalists.
Merchants, Samurai, and Maidens
While The Greater Will may rely on everyone from Tarnished fundamentalists to spurned demi-gods, The Frenzied Flame keeps things a bit more lowkey in terms of its followers and conduits. The first hints of The Frenzied Flame you're likely to find in your adventure are an item called "Shabriri Grapes" which, despite the name, aren't fruit at all but rotten eyeballs plucked from the sockets of (mostly) willing donors looking to give their "grapes" to maidens. You'll eventually be able to give these grapes to one such maiden, a blind girl named Hyetta who is on a quest to follow the guidance of "a distant light" that she can only see once she's eaten a grape.
Oh, and, yeah--she does think they're literal grapes. Don't worry, you eventually get to tell her what she's eating. It doesn't go well, but doesn't change her mind about it, either.
Hyetta begins her mission believing that she's going to become a finger maiden as a servant of The Greater Will but, somewhere along the way, decides that she's actually better suited to a different power. She makes this choice after you feed her a "fingerprint grape"--yes, another eyeball--this time from an invading Tarnished warrior who was "scorched by the Flame of Frenzy." This Tarnished, named Vyke, also drops some items with even more clues, talking about how he was once very close to becoming Elden Lord but ventured "deep below the capital" where he was burned by the flame, leaving finger print marks melted into his flesh and armor.
Vyke himself can be found at a whole corner of the map devoted to The Frenzied Flame, in the northern corner of Liurnia, where a village has apparently succumbed to the Flame's influence wholesale. They even have a guard tower that will inflict madness on anyone approaching, presumably to both guard their little community and grow their numbers. It's also worth noting that the church within the village is called, ironically, the Church of Inhibition--which may imply that the flames took root here with some deliberate sense of irony, seeing as "inhibition" is defined as the restraint against expressing an instinct, or, you know, the exact opposite of being "frenzied." If this is the case, it points even more strongly to the possibility of The Frenzied Flame and The Greater Will being, more than any other Outer Gods, diametrically opposed enemies or rivals. A sort of eldritch Batman and The Joker, if you will--as opposed to the other big powerful forces around the Lands Between that seem pretty happy to do their own things or pursue their own goals.
The residents of the Frenzied Flame Village aren't the only followers to be found, however. Eventually, if you follow the questline given to you by Yura, a samurai and Bloody Finger hunter you first meet in Limgrave, you'll be able to meet Shabriri himself--the guy who named the grapes, and apparently, started the Frenzied Flame fad. In addition to the grapes and the incarnation, he also has a talisman named after him--Shabriri's Woe--which states that he had his eyes gouged out as punishment for "the crime of slander and, with time, the bright flame of the frenzy came to dwell in the empty sockets."
If you couldn't tell, the eye theme is pretty prominent here.
Shabriri doesn't seem to have a physical form anymore, and instead, manifests within the corpse of Yura, which he's apparently reanimated to speak with you and once again impress upon you the importance of venturing below the capital to find a well hidden secret. The incentive he offers you is the safety of Melina, explaining that if you embrace The Frenzied Flame, she won't have to sacrifice herself to burn the Erdtree--you'll have your own fire to get the job done.