Titan Slayer

Weapon
Thunder Lord's Decree(Varient.) This magic weapon has the same properties as a greatsword, except that it deals 3d6 slashing damage. Additionally when fighting a huge or larger creature, the Greatsword extends blue beams of light from the top to the hilt of the blade and deals 3d6 Slashing Damage, 3d6 Lighting Damage, and 3d6 Thunder Damage. To activate this trait, after landing a successful hit on a Huge, the weapon goes into overdrive and for the next 10 minutes, this effect refreshes once you land another hit against a Huge. Additionally this weapon is sentient, the entity is a old European king(Herokian) of a age long ago. This entity demands conquest and respect.   Titan Slaying. This magic weapon has the same properties as a greatsword, except that it deals 3d6 slashing damage. Additionally When landing a melee attack on a large or bigger creature with this weapon, Red beams of solid light extend from the hilt to the pommel of the blade. Depending on the size of the target struck, the weapon deals bonus damage for 2 rounds. This effect refreshes on hit. As a side-effect, when wielding this weapon you gain vulnerability to all damage from tiny, small, and medium creatures.
Large, 1d6. Huge, 2d6. Gargantuan, 3d6. Colossal(Titan), 4d6. Legendary (this item requires attunement) [Kill a creature of large size by yourself with the weapon, using a separate attack or weapon will fail the process. While not attuned the wielder gains no proficiency proficiency bonus and has disadvantage on attacks and saving throws. This weapon's magical effect "Titan Slaying/Thunder Lord's Decree" will not activate unless attuned.]

" It was larger, thicker, heavier. And cruder than any normal blade... By all accounts, it was no more than a hulking mass of Iron." -Guard witness.
Dimensions: 6.5ft x 0.5ft x 0.25ft

Cost: 2,000gp(Varient:10,000gp) Weight: 300Ibs(400Ibs if hardcore)