Eye Injury

A cut through the cornea probably would result in blindness with the eye looking strange, but the eye would still be there. If that happened now, then there is a chance that vision could be saved, if the victim was operated on soon enough. If a knife cut through sking, then it probably would cut through the cornea. The kind of slash that you described almost certainly would result in the cornea being cut through, but a deep set eye might not be badly damaged, and an eye that was not deep set might get cut through the iris.

There's plenty online about scratches, but I didn't see anything about slashing through the cornea.

Corneal damage is my first thought, too. There are certain variables that need to be considered if only the eyelid and the cornea are cut, the main variable being the angle of the dagger. Since the slash begins at the forehead, slices through the lid and damages the eye, and crosses the cheek, the brow and cheek bones could possibly limit the damage to only the lid and cornea.

Disclaimer: I am not a doctor. The following comes from my own personal experience and much depends on how severe you want his vision loss to be.

The immediate effects of severe corneal damage would excruciating pain in the eye, inability to tolerate light, and complete loss of sight in that eye. The eye would look a bloody mess but blood wouldn't be flowing from the eye itself. If the slash crosses the pupil, scarring of the cornea would likely occur there, permanently decreasing vision. The scarring of the cornea would be worse if infection--bacterial and/or fungal--set in, and it can in as little as 24 hours. Healing time if the infection is stopped, and for the eye to look normal, could take from 6 weeks to 3 months. Again, a degree of vision loss is probable due to the scar over the pupil. For instance, I can tell you that without contact lenses or glasses a stop sign at a distance of about 12 feet is an irregular, red blur--could be someone's jacket or a flag for all I know. Small objects such as the letters on a keyboard can't be seen clearly and in sharp relief at all even from a distance of 3 inches. I can make them out, but they are hazy as the scar is centered on the pupil and I can see around it to a limited degree. If the scar was larger, encompassing most or all of the pupil, the vision loss would be far worse. If your character is adept at protecting his blind side, it's likely he could hide his blindness in that eye after healing as it would look normal and the undamaged eye may strengthen.

If the dagger penetrates past the cornea and infection sets in, as it would, the complete loss of the eye is probable and could also be a life-threatening situation. Again, I'm not a doctor but I'm guessing if that happened he'd keep the eye patch.

hth

You're very welcome, and I'm glad it was helpful. I'm kinda sorry I know anything about it, though, if that makes any sense.