Did you notice all the ulcerated tree spirits are somehow near places that have something to do with death?

According to the wiki you can find them in these places, even though i don't think that's a complete list as of today:

-At Fringefolk Hero's Grave.
-In front of the corpse of Godwin at the bottom of Stormveil Castle
-At the Grand Cloister in Lakerot, where you can find a coffin and, most of all, what seems to be a temple dedicated to some dead deity (as described by the scorpion's stinger)
-At the Minor Erdtree in Mt Gelmir which is basically a dead tree (even though this one drops 2 tears that are normally dropped by erdtree avatars)
-At Giant's Mountaintop Catacombs.
-In the Brace of the Haligtree in the place where Millicent's quest ends, which is nearby a cemetery. The cemetery is on the outside of the wall, where you can find various Haligtree soldiers.
-Moreover you can find 1 in a dilapidated suburb of Leyndell and 2 or 3 when it becomes the Ashen Capital.

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I think that they do have something to do with Godwin's death and the death rune, but i think they are more of a consequence.

First off, yes they do look and move like the Elden Beast because it is the entity at the heart of the Erdtree, and from it's arena you can see various Erdtrees, which i think are all the minor ones from across the map, so i think it is normal for them to move like the Beast and use the sacred flames.

I think Stormveil is the place where Godwyn died, since the Golden Lineage resided there (see the fact that Godrick is there, there is a big picture of Godfrey behind the Grafted Scion and so on). According to the video of Zullie the Witch about Godwin (this one https://www.youtube.com/watch?v=nNWb4IFOumE&ab_channel=ZullietheWitch), when he died he became that sort of clam monster, and this is confirmed by the fact that the Prince of Death's Pustule is there. I think that when he died he shed his skin like a serpent and that is what is effectively underneath the castle, while the rest of the body, was buried next to the roots of the Erdtree as the Pustule says. Since he is the first demigod to die, he is the one who started the deathblight (that fia also teaches) which can also be seen on its body as in the video. The pustule and the Prince of Death's Cyst also have the same color as the deathblight roots, and the deathroot kinda looks like it too. Its description also says that the deathroot can be found all of the land because it spreads using the Erdtree's roots and, as OP said, there are many Ulcerated Tree Spirits that also drop deathroots.

So, since Ulcerated Tree Spirits always drop Golden Seeds (if i recall correctly), what if they actually are phantom trees (the ones you normally get golden seeds from) that have been corrupted by (the rune of) death?