I am trying to get a German translation of the following adverbs:
Unfathomably Terribly
I am told that the suffix "lich" would be added to both("unergrundlich" and "schrecklich") however I do not know if this is correct.
Well, both of those are real words in the dictionary (see unergründlich and schrecklich in Linguee), so yes, that is correct.