U'kush, Rapier of Shock

In ages past, the Principality of Ulek was but a spattering of dwarven fiefdoms.  The Great Duke Under-the-Mountain ruled the largest of these holdings with a stern eye, a glorious beard, and a bloody hammer.  His might and and bearing did little to dissuade a group of drow, enthralled by the growing power of elemental evil, from stealing away his only son and fleeing back to the Underdark with some fell purpose in mind.  The Great Duke, unable to recover his sole heir and distraught with grief, feebly perished within the month of the disappearance (though some say that if you found his tomb, you would find his beard perfectly petrified and preserved in his coffin).

The duty to find the Duke's son fell to his noble wife, the Grand Duchess.  Being slight-of-beard, but possessing a keen insight, the Duchess spread word that a great boon of magic would be granted to anyone who would take up the noble quest to find her son, and a piece of the fiefdom would be the reward for his safe return.  Dwarven heroes and heroines flocked to prove their mettle and take up the quest, but the Duchess was shrewd: she knew that any dwarves granted land inside her fiefdom would be a future threat to her family's rule.  With a stroke of cunning, she selected the only heroes that could undertake such a quest but not threaten her line: the only five non-dwarves that offered their allegiance.

And so, "The Five Cousins" were commissioned and crafted.  These five dwarven-forged weapons were to be of the finest quality and imbued with great magic.  Alas, since the wielders were not dwarves, they would not be granted the honor of carrying traditional dwarven armament.  This did not diminish the quality of the weapons however.  Each weapon was magically imbued with a unique element; surely, if the elemental cultists worshiped four elements, a theory of five elements working in unison would be more powerful!  Five is greater than four after all....

History does not tell us the fate of the heir, nor does it tell us what became of the "Cousins" after the heroes' descent into the Underdark.  "U'kush, the Rapier of Shock", was the fifth of the "Five Cousins", and was said to be wielded by a halfling brigand of notorious charm.  Note that in the drawings of U'kush that survive, dwarven-halfling prejudice is evident: while the blade functions as a rapier, it is smaller than a typical model and it is influenced by short sword design.

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5E game info for U'Kush, Rapier of Shock:

• Required level for attunement 5th: While the blade will radiate "transmutation" magic when examined by Detect Magic, it will not function as a magical weapon unless wielded by an owner of 5th level or greater (attunement is not required for this feature).
• Attuned @ 5th Level: The blade counts as a magical weapon.  It also deals an additional +1d4 lightning damage when it hits with a melee attack.
• Attuned @ 9th Level: The blade also grants the attuned the ability to cast "Witch Bolt" as a 3rd level spell.  You must finish a long rest before you can use this feature again.
• Attuned @ 13th Level: The blade also grants the attuned resistance to lightning damage.
• Attuned @ 17th Level and Above:  Unknown.  Theories include possible sentience, possible increases to damage or damage absorption, and/or possible additional lightning-themed abilities.

PS: 5E is outstanding.  Yes, I still port everything to Greyhawk, but setting aside, WotC has a winner with this edition.