I was looking for info on Olcasglean because I think hes my favorite boss EVAR, I found this following on mythics site. Thats where I linked the lore info from.
The Celts had no concept of "Mother Earth." Instead, the universe was divided into three parts: the Middle World (the world of mortals) the Under World (the sea) and the Other World (the realm of the gods) and were bound by Bil�, or the World Tree (also the Celtic god of death, the name means �sacred tree�).
Olcasgean’s name is derived from the Irish Gaelic word olcas, meaning “evil.†It is not the creature’s original name, but rather the name given to him by the Sylvan of Hy Brasil. There have been several loose associations between ancient Irish lore and Greek myth, so some of that association was used to fashion his backstory. With that being said, Olcasgean is actually an ancient titan of Greek myth. When the titans were defeated by the Olympians, most of them were cast in to Tartarus. There were so many titans, however, that several of the lesser ones managed to escape. Olcasgean fled and ended up in Hy Brasil. Olcasgean was pursued by Olympians and dispatched, or so they thought. Originally the form of a giant world-tree, his pursuers cut down the massive tree but did not remove the roots. As a result, Olcasgean lived. While he could never leave the island, the angry and vengeful titan spread his roots out across Hy Brasil, an action that took several millennia. Everywhere his roots spread, corruption soon followed. Today, very few places in Hy Brasil are untainted by his touch.