The first of the Bs! The Balhannoth is a Large Aberrant Magical Beast (blind). “Blind” here is a keyword with some rules attached: blind creatures cannot be blinded (obviously!) and are immune to gaze attacks and other effects that rely on sight. They rely on special senses such as blindsight or tremorsense, but can’t make Perception checks beyond the range of these senses. I’d interpret that to mean they can’t make visual Perception checks.
A level 13 elite lurker, the Balhannoth has a slug-like body that menaces with spikes, five clawed tentacles, and a huge eyeless maw for a face. I’m told they first appeared in the Monster Manual IV for 3.5, but this is my first personal contact with them. They’re only on the Monster Manual and have no Vault version.
There isn’t much here. Balhannoths are indiscriminate ambush predators who live in the Underdark. Barely sapient at Int 3, they understand Deep Speech and are Chaotic Evil. Balhannoths might tail their chosen prey for hours, waiting for the best moment to strike.
The evil worm-things rarely associate with others of their own species (except perhaps to reproduce), but sometimes they can be trained to obey commands by other sapients. Telepathic creatures have an easier time of it and can “tame” captured adults. Non-telepathic trainers must train a balhannoth from birth if they are to have any hope of success.
In short, they’re pretty much tailor-made mind flayer pets, and would likely be used like guard dogs and advance forces during a raid. The flayers get the brains, the balhannoths get the rest.
Being blind, balhannoths are immune to gaze attacks and illusions and have Blindsight 10 from ESP. They’re relatively slow at Speed 4, but with spider climb they can pretty much move in three dimensions inside the Underdark. Their main mode of movement, though, is the Reality Shift, a type of teleportation that twists the surrounding space and has a 10 square range.
When balhannoths teleport, anyone standing next to them at the point of departure is dazed, and anyone next to them at the point of arrival grants combat advantage for a turn. They attack with tentacles, and can whip those tentacles around to hit everyone on a Close Burst 3 around them and slide anyone their hit freely within this area of effect. They also do increased damage against targets granting CA and can become invisible at will.
So yeah, this is how they hunt. They’ll sense you through walls using blindsense, teleport to the middle of the party while invisible, and use Tentacle Whip to hurt everyone and ruin your marching order. Then they’ll teleport away and do it again.
The suggested encounters are level 13: 1 balhannoth and 3 grimlock berserkers, or 1 balhannoth and four kuo-toa. The first are likely all servants of a mind flayer, the others had to train their pet ninja slug the hard way.
There’s not much there lore-wise, but I found the mechanics pretty cool. The concept of a teleporting ninja slug has a certain something to it. Level them up to 17 and they’d make good aboleth pets too.