As for less canonical sources, in "Eulogy for a Forgotten Hero", a Lores and Histories story removed after Demon Slayer rework and thus probably noncanonical, it appears that Silverlight was owned by an unknown adventurer who died fighting moss giants in Varrock Sewers, before Wally took the sword and used it to kill Delrith. How the adventurer had obtained the sword before being slain is unknown, but it is possible that this adventurer was the very first owner of Silverlight. Now that is probably not canon anymore, but still, food for thought...
In addition, Silverlight likely grew stronger after the Adventurer slew Agrith-Naar with it because we had summoned Agrith-Naar's true form to Gielinor. According to demon lore in RS3, demons can create astral projections of themselves to other planes while leaving their true bodies back to Infernus. Given that we explicitly summon Agrith-Naar back to Infernus, and that Agrith-Naar was said to have been an exceptionally powerful demon, it'd be possible that some Agrith-Naar's real power was transported into the sword via his blood. The dialogue indicates that the quest's experience reward is essentially a player turning Agrith-Naar's power into combat experience, so it'd make a lot of sense.
Here's the quote:
You try to wipe Silverlight clean, but Agrith-Naar's blood has fused with the blade. You heft the sword and sense that it has become more powerful. You feel the power flowing up into your arm. You can direct it into any combat skill...
Finally, some speculation. Given that no other object in-game has similar powers as Silverlight, it is likely that the spell in question is very obscure or difficult to cast. After all, there has to be some reason as to why there's only one Silverlight, instead of several Swords of Light. In addition, given the sword's association with Saradominism and its description of holiness, it is possible that the sword was created or commissioned by Saradominists.
20-Apr-2019 11:47:05