Ouroboros

Ouroboros are entities in Xenoblade Chronicles 3. The term "Ouroboros" refers to two things: giant humanoid beings of impressive strength and regenerative ability, and the s who are capable of transforming into these beings. When two Ouroboros combine via an Interlink, they merge into a single form, its appearance based on one of the two users. The Ouroboros form is resistant to damage and strong enough to take on Moebius, but can only be maintained for a limited time.

Description
Ouroboros are created by Ouroboros Stones, which when activated, transform whatever humans may be nearby with such an aptitude into Ouroboros. An Ouroboros is freed from the Flame Clock of their Ferronis, with their Iris being altered to have the image of an eastern dragon eating its tail. They also gain the ability to change their to match other people, which copies the subject's weapon and clothing, and provides an intuitive knowledge of their techniques in battle. Only six people can be Ouroboros at any one time.

Two Ouroboros can Interlink to merge into their Ouroboros form. Each Ouroboros form is unique in appearance, based on whoever of the pair has taken charge, wielding the same sort of weapon. Kevesi Ouroboros are a dark purple with more insectoid features, while Agnian Ouroboros are white with more draconic features. While Interlinked, the two participants can see each others' memories, though in a messy and jumbled fashion.

The Ouroboros form is powerful, able to take on feats of strength beyond any human, and regenerate any missing body parts as long as the core is intact. However, these feats require energy, and the more energy is spent, the more the form begins to heat up. At high heat levels, a warning sound much like a constant beeping begins to emit from the body as it glows orange. Staying Interlinked for too long beyond this point causes a buildup of black fog, and eventually an annihilation event.

Some Ouroboros traits are genetic. While the hard limit on six "true" Ouroboros remains, offspring of Ouroboros retain the symbol in their Iris and are free to select any of their choosing. There is nothing stopping them from becoming true Ouroboros under the proper conditions.

Future Redeemed
In the time of Future Redeemed, long before the main game, there is much less knowledge of the Ouroboros Stones and how they operate. As a result, their usage and results differ significantly.

Anyone who is an Ouroboros has access to "Ouroboros power", which allows them to use more powerful techniques in battle. This generally manifests by glowing a color unique to the individual, unrelated to their Iris color. When sufficiently focused, an apparition of an Ouroboros form projects from the user to aid in their combat.

Due to these weaker effects, more than six people can be Ouroboros at a time, and simply becoming an Ouroboros is not enough to free a Kevesi or Agnian from their colony's Flame Clock.

As a gameplay element
Each of the three Interlink pairs among the main playable characters ( &, & ,  & ) may independently combine into Ouroboros forms during combat. This changes the moveset made available to them.

The form of the Ouroboros taken, as well as the moves made available as a result, depends upon which member of the pair takes the lead. This may be swapped at will; for example, if Noah and Mio are currently in Ouroboros form, the form and moveset may be swapped between that of Noah's Ouroboros and that of Mio's Ouroboros. There are four Interlink levels, starting at 0, all the way to 3. The higher the level, the faster the Ouroboros Talent Art gauge builds up, and some arts have effects that only function at Interlink level 3. During battle, an overheat gauge replaces the pair's health bar, and if the gauge completely fills, it forcefully cancels the Interlink, rendering the pair unable to Interlink until the gauge is completely empty.

After unlocking the ability to switch forms, Ouroboros forms can also appear in s. Using the two Chain Orders of an Ouroboros pair causes that pair's Ouroboros Order to be selectable in the next round, which reactivates all valid participants when selected, but ends the chain attack.

Trivia

 * Ouroboros forms are very similar to the Gears in Xenogears.
 * The Ouroboros gameplay is similar to the E.S. combat in Xenosaga Episodes II and III respectively; being two party members in control of a singular entity.
 * The name 'Ouroboros' is a reference to the symbol of the same name: a dragon or snake eating its own tail, commonly interpreted as representing eternal cycles. This symbol is also seen in Xenoblade Chronicles 3.