Ethereal Brains begin life as sacs adrift in the Ethereal sea. Once they hatch, the brain portion is about the size of the human brain, but over time, as they feed on the memories and knowledge of other creatures, they can grow to enormous size. There are no know limits to the size of an Ethereal Brain. One as large several kilometers in diameter has been reported by several planar-traveling wizards, but the sighting has not been reconfirmed. Ethereal Brains wander endlessly in search of new and exciting input.
Ethereal Brains will latch onto any creature with animal intelligence or above and stun it with its tentacles (Save vs Stun at a minus -3). Once the victim is stunned, the Brain will copy all of its memories and knowledge… a process that takes 1 hour per point of intelligence of the creature that is its victim. The Ethereal Brain will then release the victim and it will become unstained in d6 rounds. After the “feeding” has been discontinued, the victim will be temporarily reduced 1 life-level for 24 hours. Most Brains are Lawful Neutral and do not try to purposely kill those they feed from.
Ethereal Brains are not usually malevolent and feed only for survival and an innate curiosity that drives them ever onward in the pursuit of knowledge and experiences, which its victims contain (as well as that which it gathers itself during its travels through the Ethereal and sometimes Prime Material planes).
Ethereal Brains can live potentially for millions of years, unless some other mishap befalls them. They can communicate telepathically with most sentient beings, if they so choose. Sometimes an Ethereal Brain will trade knowledge with a being, if it believes that it has an exceptionally rare bit of information will be gathered from the exchange. The willing “victim” will still suffer the temporary life-level loss. The trade time is equal to 1 round per skill level of Knowledge being traded. While the odd insane or malevolent Brain might try to steal more, the knowledge trade is usually abided by strictly, given the Lawful nature of most Ethereal Brains.
Extremely adept Ethereal Brains can cause the recipient of memories to believe they were their own. A memory recipient may also be caused to “relive” a memory experience of someone else, if the recipient is either willing or stunned by the Brain’s tentacles.
The Brains range from 1 (very young = less than 1000 years old) to 100 (ancient) Hit Dice.
They will use their stun ability as a defense and some are known to have Psionic attack capability.
From the D&D Wiki:
The Ethereal Plane is coexistent with the Material Plane and often other planes as well. The Material Plane itself is visible from the Ethereal Plane, but it appears muted and indistinct, its colors blurring into each other and its edges turning fuzzy.
While it is possible to see into the Material Plane from the Ethereal Plane, the Ethereal Plane is usually invisible to those on the Material Plane. Normally, creatures on the Ethereal Plane cannot attack creatures on the Material Plane, and vice versa. A traveler on the Ethereal Plane is invisible, incorporeal, and utterly silent to someone on the Material Plane.
The Ethereal Plane is mostly empty of structures and impediments. However, the plane has its own inhabitants. Some of these are other ethereal travelers, but the ghosts found here pose a particular peril to those who walk the fog.
It has the following traits.
▪ No gravity.
▪ Alterable morphic. The plane contains little to alter, however.
▪ Mildly neutral-aligned.
▪ Normal magic. Spells function normally on the Ethereal Plane, though they do not cross into the Material Plane.
The only exceptions are spells and spell-like abilities that have the force descriptor and abjuration spells that affect ethereal beings. Spellcasters on the Material Plane must have some way to detect foes on the Ethereal Plane before targeting them with force-based spells, of course. While it’s possible to hit ethereal enemies with a force spell cast on the Material Plane, the reverse isn’t possible. No magical attacks cross from the Ethereal Plane to the Material Plane, including force attacks.