Also, the back cover contains the text "The cult is attempting to harness the souls of these deaths to summon Abaddon the Demon from his dark prison deep in the depths of the Underdark." I am pretty sure Underdark is one of names that is not free for use under the OGL. The back cover is the only place it shows up in the adventure, so I'm guessing this was supposed to be scrubbed but got missed. Could be a costly mistake.
Is this an OSR or a 5e product?Yeah, that's a disallowed word.
The 5e SRD declares underdark to be PI, so if you use the 5e SRD as OGL in a product you cannot use that PI term in that product without permission.
If you use a different SRD, such as the 3.0 or 3.5 ones which do not include Underdark as a PI term, and do not use the 5e SRD, you would not be prohibited from using such a term.
Most OSR products do not use the 5e SRD.