I was thinking of a homebrew magic weapon to give one of my players, and wanted to see if anyone that it would be unbalanced or broken. It is a longsword that is black and white, split down the middle. If they hit with the dark edge, it would deal an extra 1d6 necrotic damage. If they hit with the white edge, it'd deal an extra 1d6 radiant damage. They would have to specify before rolling their attack roll, which side they would use.
I was thinking this would be an uncommonuncommon rarity weapon because it deals half as much damage as a flame tongue, but it could be bumped up to rare because of the two elementaldamage types. I would have the player find this by defeating a boss somewhere between 5th and 7th level, depending on the players.
Is something like this unbalanced?