Retrieve the Amulet of Kings.
The Vestige uses this amulet to defeat Molag Bal. It requires a sacrifice to use, and grants the power of Akatosh to the Vestige. It allows the Vestige to shoot beams of energy, which are powerful enough to injure Molag Bal.
When Molag Bal is defeated, Meridia mentions that Abnur Tharn took the amulet as it fell during the battle. She says that he will attempt to use it, but to no avail (Tharn confirms this in A Rage of Dragons EW).