Saradomin's light is an item obtained by killing Commander Zilyana in the God Wars Dungeon. It can be used to permanently remove the darkness in Zamorak's Fortress in the God Wars Dungeon, which in turn allows the player to see better and more clearly. However, doing so will not prevent the prayer drain from occurring when players enter Zamorak's Fortress.
Attempting to consume it will cause a message in the chatbox that states As you commune with the holy star, you feel the light of Saradomin preparing to fill your mind and enlighten your vision. If you submit, it will help you see through the darkness of Zamorak's evil.
If players proceed with letting the light in, a message appears in the chatbox, stating You submit to the light of Saradomin. Zamorak's darkness will henceforth have no effect on you.
It can also be attached to a staff of the dead to create a staff of light. This process is irreversible.
A player submits to the light of Saradomin, consuming the item.
For an exhaustive list of all known sources for this item, see here