This item appears to be a sword hilt.
Blade of Radiance. While grasping the hilt, you can take a Bonus Action to cause a blade of pure radiance to spring into existence or make the blade disappear. While the blade exists, this magic weapon functions as a Longsword with the Finesse property. If you are proficient with Longswords or Shortswords, you are proficient with the Sun Blade.
You gain a +2 bonus to attack rolls and damage rolls made with this weapon, which deals Radiant damage instead of Slashing damage. When you hit an Undead with it, that target takes an extra 1d8 Radiant damage.
Sunlight. The sword’s luminous blade emits Bright Light in a 15-foot radius and Dim Light for an additional 15 feet. The light is sunlight. While the blade persists, you can take a Magic action to expand or reduce its radius of Bright Light and Dim Light by 5 feet each, to a maximum of 30 feet each or a minimum of 10 feet each.
Proficiency with a Longsword allows you to add your proficiency bonus to the attack roll for any attack you make with it.
This weapon has the following mastery property. To use this property, you must have a feature that lets you use it.
Sap. If you hit a creature with this weapon, that creature has Disadvantage on its next attack roll before the start of your next turn.
Notes: Bonus: Magic, Weapon Property: Finesse, Replace Damage Type: Radiant, Damage: Radiant, Versatile, Sap
LIGHTSABER
I just now noticed this, but the orcish paladin on the paladin class page is wielding a Sun Blade. That's very cool. Always a fan of characters in D&D books being depicted using magic items that're also in the books.
Anyone else bugged that Sunblades don't have the light property?
Yeah, with the new rules, two of these can't be dual-wielded... Shame.
Well, you couldn't attune to two of them, anyway, so dual-wielding Sun Blades isn't really a thing in either version.
Yeah.. .But if you got the Sunsword from Curse of Strahd, you absolutely could do it then! At least under 2014 rules.