The Lost Temple of Forgotten Evil is an adventure from Dark Naga Adventures who seems to be the author, L Kevin Watson, though it's published by Fat Goblin Games. The name is a bit of a mouthful, clearly going for the old school naming method of The _____ (dungeon type) of ______ but perhaps having one too adjective too many. It's actually a dual statted adventure, both for 5th Edition and a generic "OSR" which is clearly AD&D, but possibly more 2e than 1e based on the power level, as the opponents have a lot of hit points. In any event, it's aimed at 4th to 6th level characters.
The Forgotten Evil in question is perhaps not so forgotten to most readers, it's Hastur, of Robert Chambers and H.P. Lovecraft fame. Or perhaps more Hastur as envisioned by August Derleth. Not the surreal, decadent Hastur of Chambers, nor the unspeakable and largely undefined horror of HPL. Instead, it's just a generic cult that grabs people and sacrifices them.
