Source: Dungeon Master's Guide

Weapon (longsword), legendary (requires attunement by an elf or half-elf of neutral good alignment)

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.

A moonblade has one rune on its blade for each master it has served (typically 1d6 + 1). The first rune always grants a +1 bonus to attack and damage rolls made with this magic weapon. Each rune beyond the first grants the moonblade an additional property. The DM chooses each property or determines it randomly on the Moonblade Properties table.

Moonblade Properties

d100 Property
01-40 Increase the bonus to attack and damage rolls by 1, to a maximum of +3. Reroll if the moonblade already has a +3 bonus.
41-80 The moonblade gains a randomly determined minor property (see "Special Features" in the Dungeon Master's Guide).
81-82 The moonblade gains the finesse property.
83-84 The moonblade gains the thrown property (range 20/60 feet).
85-86 The moonblade functions as a defender.
87-90 The moonblade scores a critical hit on a roll of 19 or 20.
91-92 When you hit with an attack using the moonblade, the attack an extra 1d6 slashing damage.
93-94 When you hit a creature of a specific type (such as dragon, fiend, or undead) with the moonblade, the target takes an extra 1d6 damage of one of these types: acid, cold, fire, lightning, or thunder.
95-96 You can use a bonus action to cause the moonblade to flash brightly. Each creature that can see you and is within 30 feet of you must succeed on a DC 15 Constitution saving throw or become blinded for 1 minute . A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success. This property can't be used again until you take a short rest while attuned to the weapon.
97-98 The moonblade functions as a ring of spell storing.
99 You can use an action to call forth an elfshadow, provided that you don't already have one serving you. The elfshadow appears in an unoccupied space within 120 feet of you. It uses the statistics for a shadow from the Monster Manual, except it is neutral, immune to effects that turn undead, and doesn't create new shadows. You control this creature, deciding how it acts and moves. It remains until it drops to 0 hit points or you dismiss it as an action.
00 The moonblade functions as a vorpal sword.

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.