"Forgive this old one that doesn't heed your warning earlier..."
Rex squints his eyes upon hearing what Arnulf said, he doesn't remember what he did but surely the Invincible Apparation sensed Arnulf approaching and gave a warning or something.
While the others know exactly what Arnulf referring to.
Just when the berserk Rex was about to kill Issac, he suddenly glanced to the side and lets out a powerful roar which was supposed to be a warning for Arnulf. Rex was distracted so much by Arnulf's presence and doesn't even realize Evelyn is sneaking up on him, this just shows that even the berserk Rex doesn't view Arnulf lightly.
Arnulf then takes a couple of steps forward walking out of the darkness.
Now that Arnulf's figure is exposed completely, Rex and the others find his entire body radiating with sharpness. His body is covered by many wounds, most of which are slashing wounds that should have come from a sword.