Because you are your mother's son, and you have inherited the work that she has done. Both the work and the blame. –Wurmalde forcing Tom to give her the key to the trunks, The Spook's Battle
Wurmalde is the one responsible for The Fiend's release. She also is the mistress of Magistrate Nowell, who is just a toy to Wurmalde's plan. She may have once been immortal like Mam, as Tibb said. Using glamour and fascination to bend Nowell to her whim. She is also responsible for gathering the three clans together and summoning The Fiend into the world. But she met her fall by the mob of men after the ceremony. She lived through so much but died from A few 10 men sad...
Wurmalde, like Mam, comes from Greece and has a bitter rivalry with her. In the past, she tied Mam up with a silver chain and nailed her to a rock so she would die. This is how Dad came upon Mam on his travels to rescue her. They might have worked together, but when Mam crossed to the light they became enemies. Their rivalry may hint that Wurmalde is a servant to Ordeen. Another reason why they might have been friends is that Wurmalde hates Mam then Mam hates her which could mean she left her alone in the dark while Mam crossed to the light. Seeing that she was alone she goes and attaches herself to other clans, possibly hoping for her vengeance on Mam.
As Wurmalde plans her vengeance on Mam she travels to The County in hope to summon The Fiend into the world. Her only goal to take her vengance on Mam. She makes her home up at Magistrate Nowell's mansion. Bending him to her will by using glamour and fascination, and keeping him fooled by telling him witches don't exist, while she makes her plans. She is succesful in gaining the Malkin and Deane trust but unsuccessful in earning the trust in the Mouldheels. But when Tom betrays Mab Mouldheel, she tells Wurmadle she will take part in the ritual. After the ritual she is captured and slayed to stop her from anymore of her sinister plans.
Wurmalde seems to be one to hold a grudge, and always is determined to achieve her goal no matter the consequnces and how dark it is. She also is very reckless and does not think her plans through and how it may cost her afterwards and is not afraid to take away lives to continue her goals.
Lamia or Not?
Since Wurmalde comes from Greece many may think that she is a lamia witch. But there is no proof of this. It is never mentioned if she was a lamia or if she had scales on her spine. And remember Mam is the mother of all lamias, and whether or not she made it public to her, lamias seem to live together like the Pendle Witch clans, but then again lamias seem to travel together in feral state than in their domestic form.
Wurmalde was killed by a vaegnir lamia witch. She was dropped on the Gore Rock at a great height.