Keltham might not suspect the dozens of other women impregnated by way of his seed stolen from out of Meritxell's mouth beneath cover of Invisibility. Or Keltham might suspect that too, not from evidence, but from it being an obvious thought. Or from remembering, once his Intelligence was raised high enough, that he'd had that thought himself in "Isidre's" presence.
Some of those impregnated women had been Baronesses of Cheliax and priestesses of Asmodeus, who would raise any child to be destined for Hell.
Some had been peasants, so that Keltham's children by them would grow up in Chelish destitution.
And some of those anonymous peasant women had been seized namelessly and returned namelessly, under threat if they broke silence, origins and destinations going unrecorded by any part of the Crown.
It wasn't a hostage-taking, in contravention of Asmodeus's orders; they weren't expecting to need hostages, then. Rather, by those tactics they had essayed to make Keltham's story be irrevocably a dath ilani tragedy, beyond even Cheliax's power to undo and give a happy ending, for any threat.