Building a Lamia Harridan

[Heavenfluff]

I am trying to design a custom Lamia Harridan for my adventure, but I am trying to make sense of both the template and the stats for one. The template stats that a Harridan needs to have ten levels of a divine spell casting class to be able to use the template, but a normal lamia is a CR 6 monster, while Harridans are CR 12. Does the Lamia receive any of class features from the class levels they get prior to becoming a Harridan, or how exactly are you supposed to apply the template?

[Eltacolibre]

Guess you didn't read how to advance monsters...ah well going to give you a quick lesson then:

Monsters have roles...Lamia the regular ones are combat monsters, which means that everytime you add a class which increases their combat abilities, they get cr+1. But if you add another class which isn't a combat class like a spellcaster, its 2 levels of the class for +1 CR until you match the original cr then it becomes cr+1 per class levels.

So in that case Lamia cr 6 + 10 levels of Cleric (CR+5) + Lamia Harridan template (CR+1) = CR 12.

For more details read more about the rules on advancing monsters. As you would also add stats for making the lamia heroic.