I was looking for the same answer.....its proper hard. I kept killing all the guys thinking something might happen but its not. I think that its either to do with that massive worm like creature, or to do with Blackwell, who changes into some kind of monster at moments but i dont know what sets him off.

It took awhile, but I eventually made it to the end of The Suffering: Ties That Bind. And,
like a lot of people, I was nearly driven to my own insanity trying to get through the
last fight with Blackmoor.

Since there doesn't seem to be a lot of advice available for how to get past the bugger, I
thought I'd post the way I did it here.

(I've heard from other places that you can one-shot kill Blackmoor if you've gotten the
final level of the good or evil demon special attack. I didn't manage to do this, so I
have no idea if it's true or not.)

At the beginning of the last battle, a Horde worm drops a random boss mob type in random
corner of the area. There's three types it might be; 1) Arsonist 2) Gun Spider 3) Slayer.
BEFORE the worm plops down the first mob, do a quick save. Since the type it'll be is
random, just keep reloading until you get the Slayer type.

Use the sawed off shotguns (there's a supply crate with them in the room just upwards from
where you start) to blast the minion slayers and dodge Blackmoors tackle attacks until
you've built up full insanity. Look for Blackmoor and run towards him. When you're close,
transform to demon and slash him (or use your special attack. I had the good side
multi-gun special attack, and whenever I tried to use it on him, he'd just blast me with
his drain attack.).

IMPORTANT: Don't kill the Slayer Boss. If you do, the horde worm will drop another boss
type, and the slayers are by FAR the easiest ones to deal with. Leave the Slayer boss
alive, and just go after blackmoor. Keep hitting him while in demon form and use the
minion slayers to recharge.

It takes awhile, somewhere between 10 and 15 normal demon attacks on Blackmoor to finish
him, but as long as you keep moving around and make sure not to kill the Slayer boss, it's
not that hard.

After enough hits, old Blacky will give in. Go upstairs and poke him. You win.