The ultimate antagonist in my campaign is called Hydra; a huge and powerful creature from beyond time and space. It is a sight too terrible to behold. Huge tentacles reaching down from the darkness that covers the cavernous roof of the Eternal Depths, where those that died at sea ultimately go. It speaks directly into the minds of mortals, and reminds them their souls are but a tasty snack to this god-like creature.
Although my players want to defeat this evil, I honestly don't know how they can succeed at this task.
Can I offer a potential solution?
The Gods are old. They struggle to follow the advancement of society and lives. While Artificers have been around for decades, no God claims their craft as it's own. Oh, there may be deities of invention. But the blending of magic and science is so new to them as to be opaque. They may understand it as well as a peasant with no interest might learn magic from his child. Slowly, and only with great enthusiasm from the child.
The Old Gods of the Forest are older still. Ancient beings whose existence is shadowed paths, sun-lit meadows, and the cycles of life and death. To them, a fence is mere underbrush. A house is a deadfall of limbs and trees. For that is all that they can imagine these things to be, lacking the concept in their ancient lives. An axe is nothing to them until the blade bites into the tree. Then it becomes known as danger, as death, as the fangs of some strange predator.
The Ancient Ones, those from beyond the stars, understand nothing of our world. Their minds are alien, containing patterns no mortal could ever hope to understand. But if you can find the edge of a pattern, the merest flicker of understanding, without losing yourself to unknowable madness, you can trap them in their own cycles. Cycles that they will not break. Can not break. Because the concept of escaping from such a thing has never occurred to them.
This is how the Gods trapped the Ancient Ones in the first place. And now that the cycles are broken, a new cycle must be created, a new maze for their impossible minds.
And then give the players the chance to learn complex information about the Hydra through the minds it's touched. Occultists and Madmen, fools and dreamers, as a way to learn some -aspect- of it's thought, so they can create a symbolic ritual, one which relies on Representational Magic, to contain the ancient thing. Not because some great wall of reality has bent to contain it. But because it will -believe- itself to be trapped... and thus be trapped.