Grafted Scion

An unnatural amalgamation of limbs that nevertheless possesses great skill with the sword.

Information and Speculation

There are several in the world; the first resides at the Chapel of Anticipation and is the 'named' boss. One appears in Castle Stormveil , scuttling around in a dining hall. Two guard the Erdtree's Favor in the Fringefolk Hero's grave. One is pretending to be (or cursed to appear as) a lobster near the Rose Church on Lake Liurnia (he is holding a larval tear so may be trying to make his way into Raya Lucaria to be reborn? a reasonable wish for a person grafted against their will), and another guards the ascent to the site of a battalion of madness-stricken Lyndell soldiers on the rise to Mt. Gelnir.
Wields the Ornamental Straight Sword

Slender straight sword patterned after an antique ornament. Superior swordsmen prefer to wield one in each hand.
After falling from grace, the dregs of the golden lineage sought power and purpose in the past.

and the Golden Beast Crest Shield

Shield of dull gold with a beast engraved as its crest. Lighter than most greatshields, and subsequently easier to wield
The beast depicted is Serosh, aged counselor who guides the golden lineage.

As a grafted creature, we can assume they are the product of either Godefroy the Grafted or Godrick the Grafted's predilections.
Given the name 'scion' (def: 1. a young shoot or twig of a plant, especially one cut for grafting or rooting. 2: a descendant of a notable family.) and that loot, it's possible whichever 'the Grafted' is responsible used their own offspring or nobles related to the golden line ("[seeking...] purpose in the past"), or their own flesh grafted with that of others. In (plant) grafting, the scion is the 'upper part' of one plant that is grafted onto the lower (rootstock) of another.
As we can see in Zullie's close up of the model on YouTube, the head ('upper part') is a young person wearing a simple crown. At the very least, the scions's heads (and presumably minds) are from nobility, but combined with the loot, speculating on their relation to the golden lineage seems safe. The scion (at least, the one at the chapel) also wears a necklace with a very Stormhawk-like wing on it, which suggests a relationship to Stormveil nobility going even further back. It could be a butterfly wing, but the proximity of the Stormhawk ashes to this scion makes more sense with the context of a stormhawk wing.
Additionally, if the scions ARE part of the golden lineage, grafting them may be a handy way to keep them from putting a legitimate claim on the throne.
We also learn from Roderika that her men ended up becoming 'part of the spider'. Since nobody else refers to Godrick of the Spider, and (again thanks to Zullie's video above) we know the scion's internal name includes spider, we can assume they provided some of the stock for these creatures and not Godrick's own body. Roderika has the bearing of a noble woman (and the blonde hair of half the nobles in the game), so one might assume her men would also be of 'notable families'.

Related Historical Events

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  Arise, Ye Tarnished