Ashrender, Odachi of the Ashen Veil

Ashrender is the Veil’s final masterpiece, forged where dying elements extinguish into nothingness. It is said the blade remembers the last moments of every element it touched, shifting through stances as if alive. Myths claim its edge can sever not just matter but the stability of elemental forces, unraveling resistances and cutting pathways between moments.

You have a +3 bonus to attack and damage rolls made with this magic weapon.

This weapon has the reach property. Additionally, when you score a critical hit with this weapon, you can roll two additional damage die for the attack.

You can choose to attune to this weapon. While attuned, you gain the following benefit:
Elemental Armament.
The sword deals an additional 2d8 damage of a type of your choice: acid, cold, fire, force, lightning, necrotic, poison, psychic, radiant, or thunder. You choose the damage type when you finish a long rest, and you can change it whenever you finish another long rest.

As a bonus action, you can prematurely change the sword's elemental damage type to another option from the list above.

Elemental Overwatch.
Whenever a creature within your reach moves, it provokes an opportunity attack from you even if it has not left your reach. When you hit a creature with an opportunity attack granted by this feature, the attack deals an additional 2d10 damage of your chosen elemental type.

you can use this feature a number of times equal to your proficiency bonus, and you regain all expended uses when you finish a long rest.

Elemental Riftstroke.
When you score a critical hit with this weapon, you can create a temporary elemental rift in a straight line originating from a point you can reach, extending 15 feet in any direction that you choose. The rift pulses with your chosen elemental type and persists until the end of your next turn. Any creature that enters the rift for the first time on a turn, starts its turn in the rift, or moves through it takes 6d10 damage of that elemental type and has its speed reduced to 0 until the start of its next turn.

You can have only one elemental rift active at a time. Creating a new rift immediately ends any previous rift you created. 

Proficiency with a Greatsword allows you to add your proficiency bonus to the attack roll for any attack you make with it.

Notes: Bonus: Magic, Weapon Property: Reach, Damage: Acid, Damage, Combat, Heavy, Two-Handed, Graze