Antumbra is a cruciform dagger made from a single crystal of clear runeglass. Pinpoint stars can be seen in the blade, where they form strange, ephemeral constellations. The dagger was made for a powerful astralite assassin, who slew many enemies with it, before it was lost in the somewhere in the Bifrost.
Antumbra has the Twin mastery. Attacks with the dagger deal radiant damage, and it sheds dim light in a 5 foot radius.
Each time you hit with Antumbra, it gains a +1 bonus to its damage, applied before the damage is calculated for that hit. As the bonus damage grows, more stars in the blade flare up. When the bonus reaches +5, that attack becomes an automatic critical hit, after which the bonus damage is reset. The bonus damage also resets if you do not hit a creature with the weapon for one minute, or disperse the blade.