Darkfall Grotto is a small cave found in the Forgotten Vale that is dominated by a roaring underground river.
It can be accessed by following the river downstream in the Forgotten Vale, to then brave a waterfall. Alternately the a secret area in the Darkfall Grotto, on a tall stalagmite, and be visited using the Amethyst Paragon and the Paragon Portal.
Many dead Falmer (×7) and dead chaurus (×4) can be found in the grotto, apparently wiped out by a few Feral Falmer (×3).
Following the river downstream, braving more waterfalls, will exit in the Darkfall Passage, dropping into the large pool there, close to the Whayshrine.