The Mind-Slime is a member of the ooze family along with creatures such as green slimes and gelatinous cubes. Unlike these, the mind-slime appears to have more than an animal level of sentience, displaying what might be considered knowledge. Noted naturalist and ooze expert Professor Uaaru has studied Mind-Slime incursions on the Sorrowfell Plains, and apparently acquired a sample of one. According to her notes, she was able to get her sample to navigate simple mazes and otherwise display critical thinking skills roughly akin to those of apes.
Of course, in a wilderness or dungeon environment, the Mind-Slime is not immediately recognizable, as it appears to be just another ooze. As Professor Uaaru's experiments suggest, however, the Mind-Slime is at least somewhat sapient, and will intelligently hunt its prey. More importantly, the Mind-Slime is interested in both a food source and an intellectual source.
Mind-Slimes prefer sapient prey, apparently feasting upon both flesh and memories. Although their acidic plasm is a rather weak acid, it appears as though the creatures actually grow more acidic if enraged or offended. More startlingly, these creatures appear to devour memories as well as flesh. They are able to memorize spells devoured from magic-users, although some of the creatures have displayed the ability to mimic certain attributes of creatures they have attacked. Professor Uaaru's (admittedly few) accounts of Mind-Slimes make note of the creatures attacking foes and then displaying expert knowledge: in one account, a Mind-Slime that attacked a warrior displayed the fighter's knack for breaking limbs, while a Mind-Slime that started devouring a thief later surprised the thief with a daring sneak attack.
Professor Uaaru's notes also contain a firsthand account of a Mind-Slime attack survivor who described the sensation as "a grotesque pulling sensation, much like the accounts of patients undergoing surgery. But that's not even quite right. It's like several feelings vying for your attention simultaneously. I mean, the creature's acid is burning you, and then you have this weird pulling sensation that feels like a sinus headache and a chirurgeon's grim work. But then there's this other feeling, like a high-pitched whine between your eyes. By description, it matches accounts of madhouse patients or experimenters with Diviner's Sage; just this dull, aching buzz in your head. It's terrible."
According to Professor Uaaru's notes, some people lose the memories devoured by the Mind-Slime, while others retain them with varying degrees of clarity. The mechanism behind this discrepancy is unclear, but inquiries are ongoing.
In combat, the Mind-Slime prefers humanoids to beasts, almost always preferring intelligent targets (although it will certainly attack animals if hungry or defending itself). For example, it has been observed to ignore mounts and go straight for the riders. It will typically use Attach on a victim immediately, using Double Attack to attack the most dangerous adjacent foe. Whenever it has a foe immobilized, it will use Brain Drain whenever that power is available, using any stolen powers at its earliest opportunity.
Typically, the Mind-Slime can use a stolen power only once per encounter. The Mind-Slime uses stolen powers as if it were the creature in question, so it uses the same attack bonus, damage, and effects. To keep the creature more in line with the original Hereticwerks creature, the DM may only allow it to successfully mimic powers with the Arcane keyword, although I would probably allow it to mimic any sort of power.
Additionally, I would be inclined to say that the Mind-Slime can only steal unused powers, but that's a matter of personal preference.
If you're a real bastard DM, characters "expend" powers stolen by the Mind-Slime, meaning that they can only use them again after a short or extended rest. Naturally, this greatly increases the Mind-Slime's threat level, although it still has to successfully use Power Drain to make this all happen.