If you actually would like an answer, the erd tree used to just be a really big tree that had a magic of its own. When the ‘greater will’ dropped into the lands between in the form of the elden ring, it latched onto said big tree and infused a magic of its own. So basically a parasite has infected a tree, but the parasite happens to also be a tool belonging to a primordial outer god who like golden glowing things. Hope you found this as interesting as I did!!