Traveling into the under-dark is always perilous and never to be taken lightly. The inky and inhospitable darkness is home to monsters of the worst sorts, and all of them are able to detect you before you detect them. Dark elves and deep dragons are bad enough, but the creatures most feared by PCs come with tentacles and a hunger for brains.
While they may be the bane of players everywhere, thought-eaters are a (mad) godsend for DMs. In and of themselves, they’re a great jump scare to throw at PCs. They’re usually found in the pitch black under-dark where PCs are already at a serious disadvantage, and even the standard thought-eater comes with the ability to repeatedly fire a brain-scrambling psionic blast, damaging and incapacitating everything in its area of effect.
As fun as it is to watch PCs nervously glancing at their Intelligence saves, thought-eater’s have so much more potential to for adventures steeped in body horror. Their penchant for casually cracking open the skulls of “lesser” sentient races and feeding on their brains is a disgusting and horrifying visual, but not like having slimy, mind-controlling brain parasites burrowing into places they definitely should not be. Thought-eaters are also highly intelligent, seeing themselves as scientists and psionically enslaved creatures as test subjects. Their endless quest to modify and “improve” their slaves means that DMs can not only enjoy stitching together unique creature encounters, but NPCs, possibly even PCS, might be put under the knife. Discovering that a favorite character is now a brain in a jar is the kind of thing that sticks with you way longer than psionic blast.
Considering they bear more than a passing resemblance to a certain octopus-headed elder god, it should come as no surprise that thought-eaters also make the perfect villains for a cosmic horror campaign. The Lovecraftian trope of the amnesiac survivor of some great horror is not only a great adventure hook, but it’s also completely in the thought-eater wheelhouse. Considering that thought-eaters are native to the Void, a dark dimension completely anathema to human understanding, whatever Void-wrought horror the survivor witnessed is liable to leave PCs wishing for a bit of amnesia themselves.
But if horror isn’t your thing, thought-eaters also make the perfect fascist villains in your 5e space opera. Slave masters who scour the universe for intelligent chattel in bizarre, biological ships, they’re just begging to be brought down by a plucky band of rebels and scoundrels. Fortunately, thought-eaters come with ready-made adversaries who fit this role perfectly: the gyth. Sworn enemies of the thought-eaters, gyth can easily serve as a “rebel alliance” of sorts, providing the PCs with useful information and entertaining backstory in equal measure. Did I mention thought-eaters are known to build floating bases around a giant central brain which, if killed, will destroy the entire base? Somehow that seems important…