I was going through the profile of Be'lakor and I noticed a few problems with the way his rules are written that would make it hard to really get the most out of him.
The first problem is in the Prince of Chaos rule that reads
You can re-roll any failed hit rolls of 1 made for friendly DAEMON units within 6" of Be'lakor
The second problem is under his Psyker box that reads
He knows the Smite power and the two psychic powers from the Dark Hereticus discipline.
The problem with the first one is that if you take him as part of a Heretic Astartes army your other units would not gain the benefit of the rule. If that was to only problem I wouldn't have so much of an issue but with the fact that he uses the Dark Hereticus discipline a new problem arises. Both the Warptime and the Prescience powers only work on units with the Heretic Astartes keywords so if you take him in a pure chaos daemons army at minimum one of his psyker powers will be useless to you. You could try to take a mixed army but even then it would be difficult to make him synergize as some of abilities would work on some units but not on others and it really would be kind of a mess.
To fix this I would propose to simply change in the Prince of Chaos rule to change the Daemon keyword to the Chaos keyword instead and for his psyker abilities to add a note saying that for the Dark Hereticus discipline to replace the keyword Heretic Astartes with Chaos instead.