amatesura:

Hannibal + The Raven Stag

Will sees Cassie Boyle impaled on the severed stag head in the field on
that first day, it is actually incredibly traumatic for him. So his
psyche has amalgamated that experience in a strange way, by producing
this black stag that has raven feathers on it that are reflective of the
ravens that are picking at the corpse. So he sees the ravens picking at
the corpse on the severed stag head and amalgamates them into this
black stag of his nightmares that is really his first touch
with the evil that Hannibal Lecter is capable of. In his mind, in the
recesses of his subconscious, there is a connection being made.

– Bryan Fuller