After crossing the Bridge of Sacrifice in the southern area of Limgrame, you may have encountered an NPC who was looking for her father. If you accept their letter imploring him to leave their lost castle, you may find yourself heading south to Castle Morne. Exploring the castle walls you will find knights fighting a swarm of demi-humans. In the tallest tower is this young woman’s father, but he claims he won’t leave until he secures the Sword of Morne. If you’re wondering what on earth that is, you’re not alone.
As you work your way out of the south end of the castle and down to the water, a boss arena will greet you. Inside you will find a large beast with a greatsword named Leonine Misbegotten. Hitting it is a way of parrying, being sneaky, or pulling aggro away from yourself by cooperating from your ashes. Leonine Misbegoten will drop the Grafted Blade Greatsword, and a keen eye on the item description will reveal that this is indeed the Sword of Morne that the commander of the castle’s army was trying to protect.
Take a minute to revisit the downed soldier to tell him that you have secured the sword and he will retreat from the doomed fortress to reunite with his daughter. We recommend tracking them from where you met them near the bridge to know their fate.
https://www.gamepur.com/guides/what-is-the-sword-of-morne-in-elden-ring What is the Sword of Morne in Elden Ring?