I figured Harrow turned to Ammit and devoted himself to her, getting out of Khonshu's service that way.

And the DID seems to have pre-dated the contract, because Khonshu says something about "you said this wouldn't be a problem". (Although I'm not sure Steven's specific existence necessarily did. He may have been cultivated as Marc's eventual escape/replacement.)

And yeah, Ammit is horrifying in her sheer lack-of-fucks-given mindset. If you're designated as hers, she will destroy you. No arguments, no begging, no consideration. Just Chomp! and you're gone.