Board Thread:Dark Souls/@comment-11991696-20130711220139/@comment-18016240-20131018102449

Or it could be that Solaire is just insane and has no connection to the god of war at all.

The sunlight straight sword's description says... "This standard longsword, belonging to Solaire of Astora, is of high quality, is well forged, and has been kept in good repair.

Easy to use and dependable, but unlikely to live up to it's name."

This description indicates that while fancy, it's just a standard longsword, not really different from most other straight swords of it's ilk. For all we know that sword is just a standard issue piece of equipment to members of the Brotherhood of the Sun, and isn't anything special. There could literally be hundreds of swords identical to his, but just belong to his fellow Sunbros outside of Lordran.

As for the whole "choosing to become undead" bit, I don't think that's literally how it happened for Solaire, since he's the only undead out of all the others the player encounters who phrases it like that. What's more likely is that unlike most other undead who are probably terrified by their new state of being he accepted it since it would allow him to become part of the prophesy and thus grant him a shot of gaining his own sun.