Armblade (Sun Blade)

An armblade is a weapon designed to integrate with the forearm of a warforged. If you’re a warforged, you can attach an armblade by attuning to it. An attached armblade cannot be disarmed or removed from you against your will, but, while the weapon is attached and the blade is active, you cannot use that hand for other actions. You can spend one minute to end the attunement and remove the armblade. An armblade isn’t inherently considered to be a magic weapon for purposes of overcoming damage resistance. However, any sort of magical melee weapon could be created as an armblade, so you could acquire a vicious armblade or a vorpal armblade.

This item appears to be an armblade (longsword) missing its blade. While attuned, you can use 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 longsword has the finesse property. If you are proficient with shortswords or longswords, you are proficient with the sun blade.

You gain a +2 bonus to attack 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.

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 use an action to expand or reduce its radius of bright 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.

Notes: Bonus: Magic, Damage: Radiant, Replace Damage Type: Radiant, Weapon Property: Finesse, by a warforged, Damage, Combat, Versatile, Sap