A cloud of swarming insects fills a 20-foot-radius sphere centered on a point the [black] dragon chooses… Any creature in the cloud when it appears must make… a DC15 Constitution saving throw, taking 10 (3d6) piercing damage on a failed save...
A lot also knock people prone. Many are clearly designed to give a single monster a fighting chance against the focused alpha strikes PCs will (wisely) unleash against “legendary” monsters. Some are suitably creepy and atmospheric, such as eyes opening on solid surfaces in a beholder’s lair to fire off an extra eye-ray attack, or walls suddenly sprouting “grasping appendages.”
Teratic Tome. These range from the atmospheric to the mechanical. Some look fairly lame on the surface of things: the first time you enter a demi-lich’s lair you take 16 points of necrotic damage, a sum that will certainly keep out the riff-raff, but barely serves to slow down a party of adventurers over 4th level. On the other end of things, they can make otherwise mundane encounters far more interesting. For instance, the terrain around a blue dragon’s lair can develope dangerous hidden sinkholes. Rodents and birds within a mile of a green dragon’s lair serve as its eyes and ears. Kraken can control the weather within 6 miles of their lairs.
These are both cool ideas and, frankly, I wouldn’t object to extending them to more monsters than got them. Those of you who enjoy playing with mythic-underworld dungeons might even want to come up with lists for orcs, goblins, and similar humanoid manifestations of the evil that lurks where the sun never reaches.