OK, I checked the MMII. I'm a bit confused. Your party is average level 11. It's larger than average, so it brings the ECL of the party to 13. The base arcanaloth is CR 17. So the encounter should be "overpowering," assuming CRs can be believed. (I know they can't always).
With
fly,
telekinesis, and
magic missile at will, he can stay out of range of melee and rain death on people. If you pick a good 6th level spell for him to know (Chain Lightning, maybe?), it could be pretty scary. (he can also Empower 4th level spells. I'm not going to do the math now to see if any of those are better than a 6th level spell, and therefore worth spending a full-round action to empower.) DR 15/+3 is hard to penetrate at range and SR24 will ensure that even your 13th level PC, if he/she is a caster, will only be able to cast spells on him 50% of the time. And then there's 20 points of energy resistance to most things and immunity to acid, poison, and mind-affecting magic. He doesn't have many hp, but he's damn hard to injure.
The one thing that may be a problem is the action economy. 7 people will get to take a turn before he gets to go again. That can turn even the biggest bad into a 2-round chump. (assuming the majority of them have a way to damage him at all, of course.) But there are ways around this too.
- You could give him some minions (either real, or summoned creatures. He's got the ability innately to summon other yugoloths, plus he might have a summon monster spell on his known spells list.)
- You could give him a metamagic rod of quicken, or a belt of battle, either of which would allow him to cast 2 spells a turn (for a few rounds).
- You can cheat a little bit.

On occasion I've made a big bad particularly big and bad by letting them act twice each turn. They get a full round of actions on their initiative, and another full round on their initiative-10. This is inspired by an ability from PF's Mythic Adventures book, but taken to the extreme. Really, it's not cheating that much. It's effectively similar to making them fight two of the same guy, but with half-hp each, who stand in the same square. (If you want, you could take away this ability once he hits 1/2 his hp to simulate one of them dying, or more dramatically, flip it around, and only give him that ability once he's "bloodied" at 1/2 hp.) Fighting two of the same monster is supposed to increase the CR by 2, though I'm not sure what the 1/2 hp factor would do. Overall, you might consider this "extra turn" ability to be a +1 CR template, IMHO.
Now, I see what you mean in that he doesn't have any fancy new tricks that they haven't seen before. (The poison claws aren't even going to come into play most likely.) So we're looking at a really hard to kill wizard with a dog's head. I don't think that's anything to sneeze at, though. Especially in Ravenloft, and in the GC campaign in particular, have they seen anything like a 12th level sorcerer in action? Play it up with your descriptions, as he shrugs off the most powerful blows, and rises up into the air, unleashing a torrent of magical energy the likes of which they've never seen before.
OK, maybe they had a decent magical battle with Azalin in FtS, but Az was pulling his punches to get them to kill him, right? And maybe they battled Meredoth too, but with proper spell choice and tactics, I think you can make this feel different enough. Play up the otherworldly-ness of him and his spells, the vile pall of the energy he unleashes, the crackling arcane power that surrounds him, filling the room with the smell of ozone, and causing the hairs on everyone's arms to stand up with the resulting static, etc. If you have access to the Spell Compendium, or Book of Vile Darkness, pick some unusual spells they might not have seen before that give him a bit of "WTH?" flavor.
ETA: just checked the 3.5 update to MM2, which I never realized existed before:
http://archive.wizards.com/default.asp? ... /20030718a
15/good is even harder to overcome than 15/+3 unless you've got someone who can cast align weapon quickly and repeatedly.