Monster Manual

As well as the minifigures—and perhaps arguably even the highlight over them—the set includes multiple buildable creatures from across D&D’s bestiary to pepper throughout the set. As well as the aforementioned skeletons there’s a couple of Myconids, a Gelatinous Cube (its innards already holding a poor skeleton’s bits, a frying pan, and a gemstone), a Displacer Beast, an Owlbear, and perhaps best of all, an amazing Beholder. I would’ve bought that as a standalone set honestly!
There’s a few more D&D creatures built into the set too, like spiders (not quite giant, not quite regular sized, even for a Lego minifigure), a few black puddings, and even a living tree.