Definitely don’t picture him standing there in shock for a few seconds. Don’t imagine that split second he needs to process Rook is gone. Really, don’t delve into the way Spite came to realize it, too. Given the demon originally has no sense of space/object permanence (banter with Emmerich) and has to very quickly come to terms with the sudden grief, and fear, and unbridled rage overcoming his host.
Probably the closest they’ve ever come (apart from Illario’s betrayal) to going full demon and almost everyone was on eggshells around him. Guaranteed, Harding and Belara were keeping those enchanted arrows close. Just in case he fell too far into himself to come back as the Lucanis that they know.
The Lucanis who was willing to kill any god Rook wanted in the name of safety he could no longer provide…
Reality so utterly out of their control, again…
But can you imagine. Lucanis on the high of finally landing his shot at ghilan'nain. That moment of anxious disbelief thinking "I did it, we did it, she's dead, Rook is safe" only to have Rook suddenly ripped away just gone into nothing, the Dread Wolf carving his way out in their place. Solas gives his "price of winning" speech to Rook but it's Lucanis who was left untethered in his fear and anguish.
Imagine how feral he probably went in those hours after. Spite howling at the forefront, wings thrashing, throat hoarse from yelling. This isn't something he can fight- it's not even a cloud face, it's just nothing. A void where Rook was meant to be. Taash is too lost in their own grief to bring Spite to heel and so it's left up to Davrin and Neve to keep Lucanis from mauling Emmrich as he rages with two voices at the mage to FIND THEM. FIND ROOK. BRING ROOK BACK (they were supposed to be safe)