Mouseferatu
Hero
Okay. I'll try to keep it brief. Ish.
The derro are the creation of something far from the Far Realm. They were meant to be scouts and infiltrators, something this entity could use to try to study and understand humanoid societies before doing whatever horrible thing this particular Far Realm Entity (hereafter FRE) planned to do. (Assuming "plan" is even the right word for the thought process of something so alien.)
Thing is, because it's so alien, the FRE didn't understand the humanoid races any better than we understand creatures of the Far Realm. So the derro are
1) Almost all identical in appearance. (It couldn't recognize the visual differences between races, let alone individuals, as significant.)
2) All psionic.
3) All insane.
4) All obsessed with kidnapping and examining/experimenting with humans, and yet unable to really learn anything from their efforts.
Basically they're the fantasy equivalent of "Gray aliens" as created by a slightly more intelligent Azathoth.
(In my version, the derro are actually the second attempt by the FRE to create real-world creatures. The first, when the FRE didn't even understand the concepts of three-dimensional space or a universe with logical laws, were the modrons.
)As far as the mind flayers, I try to play them up mostly as written, but also more alien and with a fascination for humanoid emotion and sensation, something they do not understand. So I had one game, for instance, where the PCs broke into an underground chamber and found a literal orgy of sex and violence--humanoids doing all sorts of carnal things to each other, but also beating, scratching, strangling, mangling, clawing, eating each other, all often at the same time--with mind flayers hovering above, lost in near ecstasy as they absorbed the mix of emotions from their mind-controlled slaves below.
Uh, some of my games get fairly dark.
