One blade is named Yin, it is pure black throughout with a pure white stone in the pommel. The other blade is named Yang, it is pure white throughout with a pure black stone in the pommel. These two shortswords are bound together by an unbreakable chain that can extend or retract up to 20 feet long. 

The Yin Yang swords have the thrown property, with a range of 20 feet. The thrown sword always returns to your hand as long as you are holding the other shortsword.

If you start your turn under half health, you can fuse the swords as a bonus action to form either a pure black or pure white greatsword, your choice. If the swords fuse into a white greatsword, half an angelic halo appears over your head and several angelic wings sprout from your right shoulder. If the swords fuse into a black greatsword, a single demonic horn sprouts from your head and several demonic wings sprout from your left shoulder. You can choose to end the transformation whenever you regain HP, but not before. The transformation cannot be ended by dropping the greatsword. For every minute you are transformed, you must make a CON save with DC 10 + 1 for each minute you have been transformed. If you fail this save, you become hostile to everyone and must use your action to move towards and attack the closest creature to you.

While the transformation is active, the greatsword does 2d8 radiant damage (if white) or 2d8 necrotic damage (if black). Additionally, the wings sprouting from your back allow you to jump up to 15 feet in addition to your movement.

Proficiency with a Shortsword allows you to add your proficiency bonus to the attack roll for any attack you make with it.

Notes: Finesse, Light, Vex