I can't be completely positive, of course, but judging by my expertise in these subjects, I can provide a hypothesis. Magic resides in the land as well as it's inhabitants: Shar, as I understand, can see the ambient magic from those sources, which leads to the conclusion that he has a much more receptive spirit, likely caused by exposure to magic of more unnatural sources. However, as this land loses it's magic, so too does the regeneration of the unnatural magic that feeds off his own, as magic is not expended, but recycled, though in this land, the cycle has become imbalanced, absorbing much more energy than it returns to us. Therefore, he is losing the ability to see magic. I can further support this by my own experience of late: one of my eyes has lost it's flare, returning to it's unaltered state, meaning that I, too, am losing the my power, which is to see the spirits of others. If not remedied, and magic fades from the world, Shar's ability will become completely dormant. I can sustain my other eye through the strength of my spirit, but it will most likely be weakened. Regardless of this theory's validity, one thing is for certain: it is all connected to the loss of magic.