Advice on making Serpent folk Mastermind.

I am interested in running lamentations to run a game in my homebrew humanistic fantasy setting.

I am developing a small adventure set in city of Laas, one of the major cities of the setting. Laas is a fantasy version of renaissance Venice, and home to a great many secrets and intrigues.

I am toying with the idea of using a serpent folk magician and spy as the primary antagonist in the adventure.

Those with experience of Lamentation monster/NPC creations, what advice would you give me on the creation of NPCs and Monsters for a first level group of adventures.