What is the Sword of Morne in Elden Ring?

gamepur.com:

After crossing the Bridge of Sacrifice in the southern area of Limgrame, you may have come across an NPC in search of her father. If you accept her letter imploring him to abandon their lost castle, you may find yourself heading further south to Castle Morne. If you explore along the castle walls, you will find knights locked in battle against a swarm of demi-humans. In the tallest tower is this young woman’s father, but he claims he won’t leave until he’s secured the Sword of Morne. If you’re wondering what in the world that is, you’re not alone.

If you work your way out of the southern end of the castle and down to the water, a boss’s arena will greet you. Inside you’ll find a large beast with a greatsword called Leonine Misbegotten. Beating it is a manner of parrying, backstabbing, or drawing aggro away from yourself with cooperation from your ashes. Leonine Misbegoten will drop the Grafted Blade Greatsword, and a keen eye kept on the item description will reveal that this is, in fact, the Sword of Morne that the commander of the castle’s army has been trying to protect.

Take a minute to revisit the dejected soldier to let him know that you’ve secured the sword, and he will retreat from the doomed hold to reunite with his daughter. We recommend following up on them back where you met her near the bridge to discover their fate.