Alecto is the final boss of the game, where she transforms herself into a giant sea creature. After some QTEs, you will eventually come face to face with her on a platform. Once you finish the first section, Tisiphone joins in the battle.
Alecto has a few moves here to watch out for, but they're all telegraphed and easy to dodge. When all four tips glow, go to the back of the platform and double jump over the shockwave. She'll spit water at you, so go to the back, as she'll try to bite you. When two tips glow, go to the same side as those tentacles to avoid a stabbing attack. Note the spear on the ground, so pick it up and use it to help damage the beast when she's far away.
Eventually, Tisiphone will join the battle and she'll be alone for a short while. Try to do as much damage as possible to her before Alecto comes back. If she does come back, concentrate on Tisiphone until she goes away, then the battle will resume with Alecto, with it being the exact same as before. When you see the red halo, grapple Alecto and get ready for the final batch of QTEs, with a L2 + R2 hold like before. More QTEs follow and when you eventually see the red halo above Alecto's human form, grapple her to finish it for good.
