A maze of twisty little tunnels, all different. They wrap around the roots of an imprisoned grove of long-suffering treants. The suffering of the treants empowers the evil druidic war machine known as the Biomass Engine. PCs must fight the sub-soil tenders of the entangled root system (gray renders, giant awakened moles and weasels that work in an uneasy alliance, and a nest of ants) while freeing the tangles.
Next they must emerge from the ground in the center of the artificial grove and work to subdue the freed, still-insane treants without killing them (but not before their rampage damages the earthworks and magical wards surrounding the vast forest that is the druidic "fortress" from the inside). Dodging the remaining wards, even leading the insane ones across different walls and rune lines, is paramount here.
Next they must battle the returning druidic cabal, whose most powerful members arrive first with Wind Walk and other spells (even World Wave), with closer but slower junior members arriving during the fight riding on great beasts or in the form of birds and running creatures- which is a long, wearying, but surprisingly easy fight. The druids try to restore the wards and runes during the battle; whenever there are enough lesser druids present, they break off to somewhere out of reach and line-of-sight to begin rituals fo restoration, forcing the PCs to hunt them down or watch their most deadly foes grow stronger still.
And then, within a few minutes, the Biomass Engine arrives. The true final battle begins, dodging among the comatose prone bodies of huge treants, cracked and ruined sacred stones, and runes formed from circling and forked streams. Everything is a potential weapon- but everything that holds still and smells of life is a potential meal for the Biomass Engine. A thing of hunger and fiber and flesh, it devours any dying druids and their servant creatures before beginning to absorb the treants, growing ever-larger and ever-more insane. Perhaps the only way to truly defeat it is to let it eat the treants and the trees, so that enough of its mass is flammable to burn it faster than it regenerates. Perhaps the key is repairing just enough of the wards to hold it back and restore some of the party's strength. Perhaps it's something else entirely...