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The Shard of Alexander is one of the Talisman items in Elden Ring that can increase attack power when worn.
The Shard of Alexander can be obtained by completing Alexander the Iron Fist's questline - where he will ask to duel you in Crumbling Farum Azula. He can be found on the rooftops to the west of the Dragon Temple, by taking the lift sealed by an imp gate (requiring two Stonesword Keys) inside the temple up to the top.
The Shard of Alexander will increase the attack power of all Ash of War Skills by 15%.