Could also be that following centuries of digging thru Daemonheim, Bilrach's mind was so warped and strained, that despite his power ... he simply wasn't in a good spot to be a "commander".
Bilrach's single-mindedness and loyalty is one of the few things simultaneously kept him same and driven, yet eventually drove him insane.
In the few years Zamorak has been back, Bilrach being physically close to Zammy allows for the psychological trauma he's suffered to be validated, and allow him to heal.
It's been noted that Azzy started hallucinating Zaros' commands, even though Zaros didn't have a connection to him any more.
So, Zamorak is simply keeping a close personal eye on one of his most faithful, if at least messed up, commanders close by.
Moia, as the others mentioned, could simply round out the whole list of everything he is looking for in a true Zamorakian.
And if he was aware of Lucien's true goals, this could be a proverbial nail in the coffin to Lucien's legacy to show why Zamorak's way was superior (and right) and why Lucien was nothing but a pretender.
18-Nov-2017 00:50:31