Symbionts are complete creatures, generally capable of surviving apart from a host creature for at least some length of time. They are generally Tiny or smaller, and physically very weak on their own. They survive by joining with a host creature, usually granting the host certain benefits in exchange for the protection afforded by a larger body. This symbiotic relationship usually benefits both parties (though in some cases the symbiont might better be described as a parasite).
A symbiont occupies part of the host creature’s body— sometimes, but not always, limiting the number of magic items the host can use in the same way as a magic item does. Like intelligent magic items, symbionts have Ego scores, which reflect their strength of will and drive for power. Symbionts with high Ego scores, like magic items of similar sort, can sometimes take control of their host creatures.
Source: Rising from the Last War
The daelkyr create living tools and weapons, objects formed from chitin, bone, and muscle. Many of these items bond to their bearer, drawing sustenance from the creature attuned to them; for this reason, they are commonly called “symbionts.” Though symbionts are living tools, they are mechanically considered a magic item, not a creature.
This section introduces eight new symbiont items, like those in chapter 5 of Eberron: Rising from the Last War. In addition, you could present any existing magic item as a daelkyr-made symbiont by describing the item as being made from organic materials. A dagger of venom crafted by Valaara could be a chitin blade that’s warm to the touch. A cloak of the bat might feel like membranous leather, with veins running through the material. A periapt of wound closure could be a scarab that burrows into the flesh of the bearer, enhancing their blood. Even common magic items can be symbionts; an everbright lantern could be a sluglike creature with a luminescent body.
When presenting an existing item as a symbiont, the DM must decide whether to add the Symbiotic Nature feature that other symbionts have. When such an item is attuned, it can’t be removed from the bearer, even if they desire it; the only way to end the attunement and remove a symbiont is to use an effect that ends a curse. This prevents the item from being taken away, but limits a character’s free will, so it’s up to the story you want to tell. While Symbiotic Nature is a common attribute for symbionts, it’s not a requirement. A slug that functions as an everbright lantern wouldn’t require attunement. Even if the periapt of wound closure is a scarab that digs into the flesh of the bearer, perhaps it could be pried loose, or it might release its grip voluntarily if the bearer ends the attunement.
Source: Exploring Eberron