Obtaining
The Icon will appear in your Inventory upon drawing the Sword from the statue at start of a playthrough, provided Mormo was defeated in the previous run.
Use
The Icon is used as a sacrifice to the Ruins at night. Doing so will not trigger any monsters to appear as it usually would. Instead, one of the five lights around the platform will light up.
Do this 5 times (this requires multiple playthroughs) and defeat Mormo for a surprise!