The displacer cube was the simple result of a common observation that both the displacer beast and the gelatinous cube are annoying bastards to deal with. By combining the two creatures I would be able to both have an iconic monster for newer players to fight, a familiar face for older players, and most importantly an entirely new bag of tricks for the munchkins to smack their head on. Granted I think the cube or the beast are awesome enough monsters on their own, and dealing with one should be a daunting prospect. But in a hobby where so many devotees can tell you a gelcube’s HD off the top of their head, a little novelty goes a long way.
So yeah, I put together a displacer cube. My reasoning ecologically went something like this: A) if one creature developed such a beneficial mutation as the beast’s, another might as well, B) especially if this particular sub-species evolved to hunt the beasts on their own terms, C) and let’s not rule out the essence of faeriekind infusing the creature’s gelatinous mass, having a direct mutagenic effect on its offspring/spawn/mitosis/whatever. It would live in whatever areas in one’s game world were infested with fey creatures, since these tactics would be effective against similarly elusive creatures like gnomes. It would basically lie in wait in a high traffic area (clearing, jungle growth, lakeside, temple full of cultist idiots) and set on its prey before it could get away. Like the beast, it would have long reaching tentacles capable of inflicting about 2d6 acid damage, so it could sweep a large area in pursuit of its favored meal.
Obviously, if a dwarf or a sparrow or a kobold or whatever wandered into its vicinity, hey, a meal’s a meal, and the cube has to warn off such interlopers who might scare away its preferred cuisine. The very idea of a gelatinous cube struck me as preposterous but then I likened it to certain anemones, which are just as effectively mindless and respond to specific impulses…displacer beasts and things vaguely beastly or beast shaped or beast weight would simply be a big ol’ dinner bell, while other impulses would be more muted. The crown of thorns starfish was also an inspiration…it’s basically just a mouth, and, sure, it could eat lots of things, but it gets around well enough and when given the option goes after one thing more than any other.
I already had a lot of creatures in my campaign that had taken some ideas from the regular GC but the displacer cube made me re-examine those as well, and incorporate them as a more vital element of the ecosystem.
When I actually got around to using him he ended up as the centerpiece in a dungeon, an abandoned monastery carved into the temple. Rumors suggested the monks had gone mad and begun worshiping Orcus or some other dark power. There was indeed a monster at the middle of their labyrinth but it was no dark god but the cube, and the monks were indeed mad but in a different manner from that which was rumored, owing to a particularly troublesome artifact that the PCs (again rolling on a rumor table I had made) thought was the key to slaying “Orcus.”
Between the displacer cube and the artifact it was a harrowing encounter to the point where, even after escaping immediate peril, the party chose to stay in the labyrinth and survive on rats while they stalked the cube, hoping to finish it off. No arch villain or dragon or Lovecraftian thing has inspired as many memorable moments on their own, nor engendered such player hatred, without even killing a single party member. Part of it helped, too, that I didn’t explain the creature. Knowledge checks revealed nothing. The creature was entirely alien. I obviously never called it a DC in my session.
Right now I’m in a contest on the D&D website. The winner gets their monster actually statted up in an upcoming release. To avoid any complication should I be fortunate enough to win, I’ll refrain from putting up my stats for the cubes here (I have OSRIC stats, Pathfinder, and D&D4e) until after things have been decided one way or another. Watch this space.
Until then, I post this as evidence that GMs, at least the good ones (which I’m not but which I aspire to be) do a heckuva lot of work and devote an awful lot of thought even to the things their players might consider extraneous or stupid. Especially to those things, really. It’s always some minor thing that wasn’t worth their time that bites them in the ass, in the end, and that never gets old as a GM or a player, and I speak from experience on both.