The Nightshade is the penultimate undead in the BXCMI line. (Ultimate being the lich as in AD&D, and finally Special for demons and such.) As such the category gathers together the best of the abilities of undead before it, and are of gargantuan size.
The Master's "DM's Book" defined three: the nightcrawler (the strongest) underground, the nightwalker striding the surface and the nightwing in the air. (This rather suggests a fourth in the oceans, although their affinity to sheer cold might obviate that. Later editions have provided a watery nightshade anyway.)
Where other undead are mere spirits stranded on earth (by choice or no), and where demons serve their own aims; the nightshades are servitors of the Sphere of Death. Think of them as Hel's apostles on Earth.
Nightshades are so badass that the final stages of second-edition couldn't resist pilfering them. They are a feature in epic play, for instance that nightcrawler in Beyond Countless Doorways.