Excalibur (5e Equipment)
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Weapon (longsword), artifact (requires attunement by a creature fated to be royalty)
This holy sword was the sword of an ancient king, gifted to him by a celestial of great power. People thought this blade was a legend. They were wrong. They have forgotten that there were many kings that have wielded this blade. There are few who know of this blade, mostly those who want to use it for their own reasons and do very cruel things (e.g., over tax, take other people's women, kill those who annoy them, etc.). The blade is golden in the hands of those who are of good, black for those who are evil.
You gain a +1 to attack and damage rolls made with this magic weapon. If your alignment is good, you deal an extra 3 radiant damage. If your alignment is evil, you deal an extra 3 necrotic damage.
Additionally, the sword has a special attack. As an action, you wield the sword in two hands and declare it's name out loud, forcing all enemies in a line (30ft long/15ft wide) in front of you to make a Dexterity saving throw against DC (9 + Strength modifier + proficiency modifier). The creature takes 4d12 damage of the radiant/necrotic damage on a failed save and half as much on a successful save. The damage increases to 8d12 if the target is of an opposing alignment. After using this attack, you gain one level of exhaustion, and you cannot use it again until you take a long rest.