Antumbra

Antumbra is a cruciform dagger hilt made from a single piece of crystal clear runeglass. As a bonus action, you can cause a blade of astral energy to coalesce from the hilt, or disperse the blade. Attacks with the blade deal radiant damage. Pinpoint stars can be seen in the blade, where they form strange, ephemeral constellations. While the blade is active, it sheds dim light in a 5 foot radius.

If you make a thrown attack with this blade, you do not actually throw the hilt, just the starry blade, which reforms around the hilt after you hit or miss. If your other hand is empty, you can use two-weapon fighting with Antumbra alone.

Each time you hit with Antumbra, it gains a +1 bonus to its damage. As the bonus grows, more stars in the blade flare up. When this bonus reaches +5, the next hit you make with it is an automatic critical hit, which resets the bonus to 0 afterwards. The bonus also resets if you do not hit a creature with the weapon for one minute, or disperse the blade.