The Rotten Crystal Sword is great for magic users who want to deal the extra Scarlet Rot damage to the enemies they encounter. This sword is exactly like the Crystal Sword and has the same move set, but comes with the benefit of inflicting Scarlet Rot on those you hit with it. Here is where you can find the Rotten Crystal Sword in Elden Ring.
To obtain this weapon, you will first need to reach Elphael. This is the area beneath the Haligtree. To reach this area, you will first need to reach the Consecrated Snowfield by using the Haligtree Secret Medallion at the Grand Lift of Rold. Once there, make your way to Ordina, Liturgical Town, and complete the puzzle there. This will allow you to teleport to the Haligtree. Make your way through the Haligtree until you reach Elphael.
After reaching Elphael, you will need to make your way to the Elphael Inner Wall Site of Grace. This Site of Grace is located in the room behind where the Erdtree Avatar is. When you reach this Site of Grace, make your way down the stairs and turn left. Jump over the roots and you will find Crystalians. You can choose to fight them if you want but they are hard to fight in such a small area. The chest in front of them contains the sword. You can easily grab it by luring the Crystalians out and doubling back around to the chest.
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