The horn might not be missing, it could just be one horn, either the other one hasn't grown yet, or that's just in his genes, but yes, maybe Darkeye had another horn that was severed in battle. I think maybe the crack on Tastidian's face is from all that energy concealed inside him, perhaps like a caterpillar waiting to bloom into a butterfly, he had so much energy in him that he might have had nearly enough to transform into a god. A second theory is that the scar appeared when Nelis (that is how it happened right?) transferred some of her energy into Tastidian, and perhaps the procedure was painful, or had some sort of effect on him, such as the scar on Abe's hands in the Oddworld series when he got his powers, that gave him the scar.
