There is a temple with its only access under the roiling blue waters of the Stojanow. The immense pyramid stretches a hundred feet upward and a hundred feet across, nearly damning the river.
Valistrae's lockpicking skills proved a match for the lock, but the glyph nearly killed their healer, Bronning. A few hours of searching uncovered signs of horrid experimentation by the mad sorceror in charge. One of the creatures, Squib, was a kobold/goblin frankensteinian hybrid, terribly twisted by magic and experiments that rendered it mute, half-mad, and undead.
The terrible wretch led the party through a set of potentially lethal traps to its master, and the pitched battle raged all throughout the pyramid, until at last the yugoloth attempted to escape by plane shifting, and the party pursued.
Or rather, deceived….
The yugoloth opened the shift, but didn't leave. Instead, it realized the party would pursue it, and so stayed behind, and began in their absence laying waste to their holdings. Arriving miles apart, it took the party days to find one another again, in the village of Corpsehaven, deep in the haunted wastes of Blood Rift.
Fizzl managed the magic to return to the prime material, and then used a gift she had received, a mighty Wish… she asked the Fates, "bring them all back!" That was when the Blood Rift intersected with the Prime, and thousands of the Forgotten opened their eyes, and were home, and Tanar'ri and other fel creatures appeared over Abeir Toril.