I want to give it a 4/5, but there are simply too many errors. The NPC group has 5 members, only 4 of them have stat blocks. The BBEG is a vampire that cannot access his own coffin. I will quote p13 directly here:
Misty Escape. When he drops to 0 hit points outside of his resting place, Rhacoph transforms into a cloud of mist (as in the Shapechanger trait) instead of falling unconscious, provided that he isn’t in sunlight or running water. If he can’t transform, he is destroyed. While he has 0 hit points in mist form, he can’t revert to his vampire form, and he must reach his resting place within 2 hours or be destroyed. Once in his resting place, he reverts to his vampire form. He is then paralyzed until he regains at least 1 hit point. After spending 1 hour in his resting place with 0 hit points, he regains 1 hit point.
Shapechanger. If Rhacoph isn’t in sunlight or running water, it can use its action to polymorph into a Tiny bat or a Medium cloud of mist, or back into its true form. [...] While in mist form, [...] He is weightless, has a flying speed of 20 feet, can hover, and can enter a hostile creature’s space and stop there. In addition, if air can pass through a space, the mist can do so without squeezing, and he can’t pass through water. He has advantage on Strength, Dexterity, and Constitution saving throws, and he is immune to all nonmagical damage, except the damage he takes from sunlight.
[3 of 5 Stars!]