A New Party
So that was how it was.
Casimir had been mind controlled at the beginning, perhaps up to the point where he set up the party event that began Aldrich's ascension into a Lich. Afterwards, he had chosen to stay out of his own volition.
No doubt, there was pressure involved. Once the party was set up and done and Casimir had indirectly caused the deaths of representatives from all three prongs of the Trident, he no longer had any support elsewhere other than Aldrich.
But had the partnership between Casimir and Aldrich been one more based on force than mutual understanding, then Aldrich had no doubt that Casimir may have already plotted something behind his back.
It was a strong reminder to Aldrich that if he wanted to go down the route of having a legion that could think for themselves, he needed to understand them.