He was fired from Superman and quit The Witcher (because the writers on the show literally hate the books).
He recently left The Witcher to play Superman again but just two weeks later studio changed their mind and announced he will not be playing Superman anymore. With WH40k series he will be on production team so it is less likely he will get dropped again.
He left Witcher to to more Superman, but then James Gunn took over DC and scrapped that. But this is exactly what Cavil needed, a big show that he is also super excited about. Cavil plays warhammer as a hobby and is passionate about it.