I have been thinking about how we've recently got Dead Messenger. That makes three energy exotics that can change their element type. I have thought about a sword that has that perk but changes its heavy attack depending on its element. I thought vortex for void, caster for arc, and slam for solar.
Idk what it should have for a secondary perk but I think for every elemental shield that is broken with the same type it will grant a small damage buff to the sword.
If it has a catalyst, I'd say giving it stasis which it's heavy will summon a wave of stasis crystals like the Titan melee attack. But costs more ammo on heavy. Or the sword would have stasis without the catalyst and just have it grant ammo on every kill you get with a challenging enemy.
I don't know about the name but I think Threadbreaker would probably suit it. To represent our will of control when we have the darkness and dedication to the light or whatever I don't know lol.