the beast was the prominent foe from PoE's....i guess early access stage of the game, big eyeball & tentacle lovecraft monster that flesh-creeped all inside a mountain. In the initial game the "eternal empire" (dead civilization. rome) had built near that mountain to refine "virtue gems" (skill gems). out of it's flesh and carapace like the ancient vaal did. The origional villians Piety and Dominas was using those gems to to try to get real powerful.
then at some point you defeat it...and i turns out sin had created the beast because it essentially steals the power of gods and makes those into gems mortals can use, instead of gods being petty tyrants over the rest of humanity. it also put sin asleep during this era. then the devs went "lol, no more cthulu monster, fight gods now". then 2 hours later the devs went "lol, just kidding, gods are chumps back to the beast" and Sin goes around with you killing the gods.
then in PoE 2 in the opening/trailer cutscene showing the count of ogham and a party of soldiers "killing" the robed one, and stealing his weird orb, it had the seed of the beast in it....but even by campaigns end we don't really understand the motivation of the people who shipping, dragging, and towing t he beast around as it re-grows. if i had to take a WILD guess their plan is to...put the gods to sleep, and then get powerful using the virtue gems. again. for the....4th or 5th time.