Scenario 1:
Horror's insecure about the hole in his skull and one day he feels really bad about it, claiming that it makes him stupid, ugly, etc. He either goes to Nightmare for comfort or Nightmare goes to him, telling Horror that it's not true. When that doesn't seem to work and Horror keeps going on about how Nightmare is smarter and looks better than he does, Nightmare makes his corruption leave his covered eye revealing a massive missing chunk of his own skull and asks Horror if it makes him ugly or stupid. Of course Horror says no and the two have a bonding moment <3
Scenario 2:
Suggest your own :D