This straight sword features a serrated edge and a sharped tip. When you make a melee attack with this weapon you may choose to deal either slashing or piercing damage - this is a physical feature, not a magical one, and can be used even if not attuned. While attuned to this sword you gain proficiency with longswords if you are not already.
This weapon is magical with a +1 magic bonus to attack and damage rolls made using it while attuned. Also while attuned the sword gains the light and finesse properties and can be treated as if a shortsword (sword stats do not change) for any spells or features that require the use of a shortsword.
The sword holds 3 charges. While attuned you may use 1 charge to cast the spell Hunter's Mark at its base level. All spent charges are recovered daily at dawn.
Proficiency with a longsword allows you to add your proficiency bonus to the attack roll for any attack you make with it.
Notes: Weapon Property: Finesse, Weapon Property: Light, Proficiency: Longsword, Bonus: Magic, creature proficient with shortswords or longswords, Combat, Versatile
Too strong for a rare weapon. If it was JUST the variable physical features and the finesse properties, or JUST the hunters mark ability, it wouldn't be so broken.
Rather than breaking the weapon rules, you could say there are variations of it as either a Longsword, Shortsword, or a Scimitar.
I'd suggest just the +1 bonus and the hunters mark feature. Maybe add a variable option to use a charge on, like gaining advantage on a single survival check.