One thing about them though - is it written anywhere that their loyalty to Ivana is part of their make-up? Could an ermordenung turn on its mistress? PCs trying to cure an ermordenung, or dealing with a loved one who has been turned into an ermordenung, or managing an ermordenung who wants to defect (to Ivan, or to somewhere outside Borca) could be interesting. Ermordenung are explicitly not immune to each others poison (otherwise the 'oh no i can never touch a loved one again!' tragedy aspect of them has a honking great loophole in it), could there be an internecine civil war among the ranks of the ermordenung? Everyone ELSE in Borca intrigues and backbites, why not these guys too?
Only vaguely
From 2e
"The ermordenung are a dark and evil people found almost exclusively in the domain of Borca. Here, they act as elite agents who serve Ivana Boritsi, the ruler of that dread domain. On rare occasions, they are sent on missions outside of Borca to further the interests of their mistress.
* * *
Habitat/Society: The ermordenung live as members of the ruling elite in Borca. They seldom mix with "the common folk" unless acting on behalf of their mistress, Ivana Boritsi. The fact that the ermordenung cannot touch another living creature without causing it to whither and die causes them endless heartache. They have been forever denied the physical pleasures—the caress of a lover's hand, the embrace of a close friend, the affectionate hug of a child—that mean so much to mortal men.
Their inner suffering and agony has been marshalled to make them cruel and heartless agents who carry out the orders of Ivana Boritsi without question.
Ecology: The ermordenung are normal humans who have been transformed, at the command of Ivana Boritsi, mistress of Borca, into nightmarish creatures.
The process by which these creatures are created is dark and mysterious, but is believed to be so brutal to its subjects that only the most physically fit can survive it. Because of her own passionate nature, Ivana Boritsi selects only the most physically beautiful of her people for the "honor" of transformation."
In 3e:
"A seductive, twisted and evil people of the domain of Borca,
the ermordenung are created by Ivana Boritsi, mistress of Borca, to become her most loyal and elite operatives and assassins. Living as members of the ruling class, they appear as normal human beings of beauty, nobility and grace. Both men and women are exceptionally tall and marked by coal-black hair, dark, hypnotic eyes and an unusually pale complexion.
* * *
Forever denied normal physical pleasures, inner suffering and agony leads most of them to become cruel and heartless, carrying out the orders of Ivana Boritsi without question. They are rarely sent on missions outside Borca."
So it does not say directly why they are loyal to her instead of bitter and hate her, but it could be the creation process, the status position, or something else.
The fact that she rarely sends them out hints at possibly not having great control outsider her domain so their could be a darklord power or a poison thing that borders block.
Also I was misremembering them as only women. From the initial 2e description it talks about both sexes as does the 3e one.