Thanks, everyone, for the suggestions. Golgot, your mention of the celestial helpfully got me reflecting that the Xthonic religion is really the opposite of celestial – it’s an earth-cult, all stones and dust and fixity.
So I’m thinking that the symbol of the Karagond Hegemony should be an omphalos – the navel or center of the world, which Karagon plainly considers itself to be. It’s drawn in its most simple form as a circle crossed with an X, but can be done in more elaborate forms with multiple crosses or spirals converging on the center. The X and spiral (helix) both have religious connotations.
As for your symbol, if your band has good morale they’ll let you pick the symbol, and otherwise they’ll pick one. 
Some of the options I’ll take away from this suggestion-fest and write into the game are:
The eight-rayed sun.
A broken chain.
A threshing flail.
A bloody hand.
Thanks again, all!