Note/Disclaimer: this item is based around a moonblade described in the Dungeon Master's Guide p.217. The particular history provided with this item is a mix of homebrew and original lore of the Forgotten Realms campaign setting created by Wizards of the Coast.

In times before the Breaching, a limited set of moonblades were created to be claimed by the elves of the multiverse. Of all the magic items created by the elves, one of the most prized and jealously guarded is a moonblade. In ancient times, nearly all elven noble houses claimed one such blade. Over the centuries, some blades have faded from the world, their magic lost as family lines have become extinct. Other blades have vanished with their bearers during great quests. Thus, only a few of these weapons remain.

A moonblade passes down from parent to child. The sword chooses its bearer and remains bonded to that person for life. If the bearer dies, another heir can claim the blade. If no worthy heir exists, the sword lies dormant. It functions like a normal longsword until a worthy soul finds it and lays claim to its power.

A moonblade serves only one master at a time. The attunement process requires a special ritual in the throne room of an elven regent or in a temple dedicated to the elven gods.

A moonblade won’t serve anyone it regards as craven, erratic, corrupt, or at odds with preserving and protecting elvenkind. If the blade rejects you, you make ability checks, attack rolls, and saving throws with disadvantage for 24 hours. If the blade accepts you, you become attuned to it and a new rune appears on the blade. You remain attuned to the weapon until you die or the weapon is destroyed. The DM determines which rune appears on the blade by choosing one most appropriate to the creature that attuned or by rolling on the 'moonblade properties' table (DMG, p.217).

One such moonblade is the Winter's Shard, owned by the ******** elven family, which were one of the few families who had such a weapon while not being of noble heritage. It has had 5 previous users before the item went missing, and therefore has 5 runes inscribed on it. 

Rune of strength. The first user was a fighter, and gave the moonblade a +1 on attacks and damage rolls made with the weapon.

Rune of alertness. The second user was a ranger during dangerous times, and gave the moonblade the ‘guardian’ and ‘beacon' minor properties (DMG, p.143).

Rune of sealing. After the Breaching, the moonblade got the ‘key’ minor property (DMG, p.143).

Rune of strength. The third user enhanced the moonblade’s combat prowess further, granting an additional +1 on attacks and damage rolls made with the weapon for a total of +2.

Rune of spell storing. The fourth rune was inscribed by a druid, granting the moonblade the properties of a ring of spell storing (DMG, p.192)

Rune of weightlessness. The fifth and final user was another ranger which relied on her dextrous movements, and granted the blade the finesse property. 

Attunement. It has the same attunement requirements as a moonblade has, including requiring a special attunement place of significance to the elven race, such as the Palace of Elvenkind or Temple of the Seladrine in the elven capital Ilynesera (homebrew content). The moonblade will show a favour to Neutral Good Elves of the ******** family.

Key. The moonblade, together with the remaining eleven moonblades in the world used in the Breaching ritual (homebrew content, the event that created a strengthened barrier between the elemental chaos and the Inner Planes), function as the key to the newly created barrier between the material plane and its surrounding planes. If all moonblades were to use this feature as an action simultaneously, the magical barrier between the elemental chaos and inner planes is weakened and prone to attack.

Sentience. A moonblade is a sentient neutral good weapon with an Intelligence of 12, a Wisdom of 10, and a Charisma of 12. It has hearing and darkvision out to a range of 120 feet. The weapon communicates by transmitting emotions, sending a tingling sensation through the wielder’s hand when it wants to communicate something it has sensed. It can communicate more explicitly, through visions or dreams, when the wielder is either in a trance or asleep.

Personality. Every moonblade seeks the advancement of elvenkind and elven ideals. Courage, loyalty, beauty, music, and life are all part of this purpose. The weapon is bonded to the family line it is meant to serve. Once it has bonded with an owner who shares its ideals, its loyalty is absolute. If a moonblade has a flaw, it is overconfidence. Once it has decided on an owner, it believes that only that person should wield it, even if the owner falls short of elven ideals.

Proficiency with a Longsword allows you to add your proficiency bonus to the attack roll for any attack you make with it.

Notes: Weapon Property: Finesse, Bonus: Magic, Bonus: Initiative, by an elf or half-elf of good alignment, Damage, Buff, Combat, Versatile, Sap