Since the Abyssal are still excavating this site, there must be another tablet they are searching for. If you can prevent them from translating that tablet, it may doom their chances of ever finding him.
In any case, Commander Calthyx cannot be allowed to share the knowledge he has learned in this place. Without their leader, hopefully the other cultists will lack the guidance to continue their search for Calyx. The time to strike is now!
Well done, [%NAME]. I fear this may not have been enough to stop the Abyssal's quest permanently, but we will be on guard in case more show up to continue the effort. Hopefully his next in command is an incompetent officer without the courage to run things as Commander Calthyx did.