This is a very cool weapon and quite powerful. Probably very rare if I had to assign it a rarity.

Strength is obviously great since you can use it with GWM. Super powerful to be able to use a shield and GWM at the same time. Also allows dueling fighting style for even more damage. Shatter defenses is also extremely synergistic with GWM.

Dexterity is kind of weak. Fun to be able to use TWF with it but no where near as strong as strength. The parry ability is probably weaker than it needs to be. You could make it work like defensive duelist easily enough (add proficiency bonus or dex mod to defense) without breaking anything. Serpent's strike sounds like either a reaction attack or a reach attack. Spend 2 charges to make a reaction attack against someone attacking you with advantage. This reaction happens before the first attack hits, potentially disrupting the attack.

Constitution is clearly strong since everyone needs con. The abilities are fun. I like it.

Intelligence is crazy powerful. Exploit weakness is just brutal on an arcane trickster. Steady aim, plus this gives basically an autocrit with sneak attack. I'd probably switch the cost of the two abilities.

Wisdom is also very strong. Ultra-instinct in particular is extremely good since it can completely negate a crit.

Charisma is fine. Charisma classes don't need a lot of help I suppose. Awe is super strong on a Sorcadin. Upcast inflict wounds, get a weapon attack and smite all in one swing!