A blade of terrible power, rumored to cut through anything.  Rumors say it's possessed by a powerful, primal spirit with an unending hunger for combat.  It has seen many hands, always being taken from one host to another.

\\Attunement level 1//
As a bonus action, you may summon Zantetsuken from anywhere to your hand.
When you make a successful hit with this weapon, you may add 1 damage, of either Radiant, Lightning, or Thunder damage.
When you fail to connect with an attack with this weapon, roll a Wisdom saving throw with advantage.  If you fail, take 1 Psychic Damage.

\\Attunement level 2// (Required Player level 6)
Successful attacks with Zantetsuken now add 1d4 damage of either Radiant, Lightning, or Thunder damage, instead of 1 damage.
Failed attacks with Zantestsuken now have you roll a Wisdom saving throw without advantage.  On a failed save, take 1d4 Psychic Damage.
Zantestuken now gives +1 to both attack rolls and damage rolls.
Once per short rest, you may focus onto the blade.  For ten rounds, Zantetsuken crits on 19 and 20, as opposed to just 20, and deals three times crit damage as opposed to two times.

\\Attunement level 3// (Required Player level 13)
When you first attune to Zantetsuken for the third level, make a Wisdom saving throw.  On a failure, your character is possessed, and an encounter with the primal will begin.  The character will not stop until slain.
 If the character is brought back from the dead after this, they can no long attune to Zantetsuken, and permanently suffer disadvantage to all melee attack rolls.

\\Successful Attunement level 3//
On a successful Wisdom save, you have fully attuned with both the sword and the spirit within.
Failed attacks no longer have you take Psychic damage.
Successful attacks now add 2d4 Radiant, Lightning, or Thunder damage, instead of 1d4.
Zantetsuken now gives +2 to both attack and damage rolls.
You may converse with the spirit of the sword for information on creatures you are fighting.
Once per short rest, you may focus on the blade.  For ten rounds, Zantetsuken swings with advantage, crits on 19-20, deals three times crit damage, and any critical hits with the blade cause the targets to automatically fail their death saves, if they have any.
Once per day, you may invoke a ritual, with 250gp worth of crystals, which are consumed in the process.  For two hours, you are possessed by the spirit of the sword while maintaining your will.  When the effect ends or is dispelled, you suffer a level of exhaustion.  If this exhaustion would cause death, the body is instead permanently possessed, and an encounter with the primal will begin, not stopping until defeated. 

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

Notes: someone with proficiency in longswords., Damage, Combat, Sentient, Versatile, Sap