Burned eyes I guess? One reason why Helmets or face protection is often given out in dangerous occupation.
Eyes are highly valuable and hard to treat when injured in any capacity.

Unless one is is the female earth bender Toph from avatar, seeing thought contact with the earth is useful but a pale imitation of the real thing.

One of the most tragic things is the ability to read and write in my oppinion. Can't type and create stories if I am suddenly blind.

Interesting development but why make one of the very view main characters basically useless? Unless it can be fixed, it seem unnecessary limiting.
Unless it was the goal to free up a place if a companion without killing of a character?

The enemy and friends of the Shield hero have lost potencial a army worth in red shirt characters, but the Shield Hero lost a useful companion and will feel guilty for the burden his companion has to endure.
Might even change his willingness to fight on the front lines.
The Shield hero and his companions are forced to fight for survival, but there is a huge difference between someone able to fight and someone willing to fight.
Best example that comes to mind on the spot is the difference between someone fulfilling only primary objectives of a mission, to someone going for secondary and other optional targets.
Everyone that played tactical games knows the difference in difficulty and cause of effect.
In games it means extrea rewards, in real life it would be minimal gain for maximal threat of permanent injury or death.
Glory and short term popularity can he a poor motivation for someone that cares little for things like that.
So long things in short:
If this is permanent in any way, Naofumi might only focus on leveling himself and his direct companions up and go for absolute only the most critical, minimal risky exposure giving objectives.

Making him from a passion filled main character, to a passive side character.

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Glad you brought that up, and you’d be right. Losing any of your senses, will undoubtably hinder you. But on your point about Naofumi and his motivation to fight, it won’t be a driving force from Melty being blind, so to speak. It will, of course be a component like with anyone who has others underneath them.

There also is a reason, that will be further revealed further down the line as to why. This was not just an accident, even if the chapter presented otherwise.

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Looking forward to the continuing of this adventure over time 👍