Carceri, the oft-forgotten prison plane located between the Abyss and Hades, has some strange inhabitants. Let's have a look at them!
All demodands share the ability to summon others of their kind (as fiends are wont to do), immunity to poison, outsider traits, and, ironically, continuous Freedom of Movement.
Farastu are the smallest, least dangerous demodands. They have 11 outsider RHD, are medium-sized, and lack the wings of their more advanced kin. Additionally, the demodands possess DR 10/Good, acid immunity, cold and fire resistance 10, and SR 23, as well as +14 natural armor and a slightly above-average land speed of 40 ft. Their natural weapons are two 1d4 claws and a 1d6 bite.
Farastu stats are fairly unimpressive, with +8 strength being the biggest boost they get. The other stats are all in the +2-6 range, with the exception of their -2 intelligence.
However, it should be noted that with a single added HD, farastu receive a dramatic boost in power. They become Large, get additional strength and constitution, and deal more damage with their weapons to boot. Outsider HD aren't half bad on their own, and with the size change factored in I'd argue taking one more is worth the class level it eats up.
Special abilities are middling. Improved Grab is improved grab, Adhesive Slime is interesting for giving a +8 grapple bonus and occasionally snatching a weapon, Scent has its moments, and 3/day Rage is a welcome buff.
Furthermore, farastus can summon 1d4 others of their kind with a 30% chance of success. It's not the most reliable ability, but a handful of moderately tough meat shields comes in useful from time to time.
Finally, the SLAs: at-will Detect Magic, Clairvoyance, Invisibility, and Tongues are noteworthy (at-will Fear less so), 3/day Fog Cloud and Ray of Enfeeblement are a waste of an action, 2/day Dispel Magic is underwhelming.
Ultimately, the farastu makes for a reasonable grappler... but it's still quite a lot smaller and weaker than many at-level opponents. To make matters worse, it can't do all that much outside of grappling, other than raging and dealing reasonable damage. 12 RHD are a high price to pay, and the resulting creature is good enough that I'm comfortable assigning +0 (though maybe -0 is better?), but nothing beyond that.