Character Analysis
Ah, the other evil in-law.
Donegild, the second treacherous mother-in-law Custance encounters, is almost a carbon copy of the conniving Sultaness. (We guess Custance just had bad luck in that department.) Of course, there are some differences.
For one thing, Donegild's methods are a little more subtle. Instead of killing everyone who gets in her way, she falsifies letters between her son and the Constable in order to get Custance abandoned at sea (again). Instead of protesting at her son's conversion to Christianity, she simply objects to his marriage to someone so different from himself. She's a bit more sneaky. A bit more vindictive, if we may say so.
The Other Mother-in-law Hates the Other
Donegild's fear of the so-called Other leads her to symbolically transform Custance into what she fears when she claims to her son that his wife is some kind of elf or evil spirit. She's off her rocker of course—Custance is the best—but she's really a victim of her own crazy fears and imagination.
And in contrast to the Sultaness, Donegild acts alone. Her evil is specific to her and not suffused throughout her whole religion or culture. In Donegild, then, we have a representation of individual rather than cultural evil. It makes sense then that Donegild dies alone in punishment for her crimes, rather than as a victim of mass genocide like the Sultaness. She pays the price and that's that.