Memory a bit fuzzy here but. I think there was mention about the balance of light and dark being different on each shard and the warrior of darkness we fought before came from a shard where the scale tipped too heavily to light, resulting in the world being entirely consumed by light while the void we learned about in ARR 24 man duties is another shard where the scale tipped too far to darkness which caused that shard to be consumed by darkness. If these balances do vary by shard then its entirely possible we may travel to a shard tipped towards light that needs a champion of darkness to re-balance it.