adding a custom mob with a custom drop

Hey! I was wondering How would I go about this? I really don’t know anything about custom mobs, and I want it to drop like a custom item that the player needs to craft something in the datapack.
And for specifics, I kind of want the mob to be similar to an iron golem, and also similar to the warden a little bit, without the airborne attacks and like not being able to see and stuff. Sort of like another boss
I am trying to sort of add a new boss that can be spawned by doing some ritual or something