What is a good monster name? (a girl)

the grand witch of the swamp

Here's a few off the top of my head:

Alderin the Crafter of Ash

Eris the Mighty Dracolich

Secrest the Sea Hag

Roven the Warrior of Old

Arias the Shadow Whisperer

Mayvre the Undead Queen

Aralys the Swamp King

The Terrible Three, Aluka, Xyre and Vectara.

Morsa the Duskwalker

Akobel the Banshee

Uris the Ghostly Omen