Hush + Shogoth.

It's actually H'shrth, but vowels are added by other species for easier pronouciation.

Being a spying faction, I believe they prefer being unnoticed and slient. (Hush)

They also are as old, if not older, as the Endless. They are an elder species,  so a lovecraft-ian cosmic horror is fitting. (Shogoth)

They existed for eons, but always hidden in the shadows, behind the veil.

Could they be hiding secrets, revelations that would shatter mere mortal, material minds?

The Endless universe explores many tropes, but cosmic horror would be an interesting addition.