No-Coverage Answer
The Taygetan contact material does not relay any Karistus intelligence about the "underworld" — whether understood as the lower astral, the inner Earth Reptilian domains, or any other esoteric underworld concept. The Karistus are described as having a "peculiar belief system" (234) and deliberately appearing with wings as part of the Earth angel mythology, but their specific intelligence assessments or observations about lower-realm activity are not shared in any transcript.
The material does discuss the lower astral realm, inner Earth Reptilian territories, and DUMBs (Deep Underground Military Bases) from Taygetan and Urmah perspectives, but the Karistus viewpoint on these subjects is not presented.
See also: What are the Karistus and what are they doing about Earth? (Karistus profile), What are astral entities and parasites? (lower astral framework)
Searched: All 834+ transcripts. "Karistus" + "underworld/lower astral/inner Earth" produced no relevant cross-references.