You have a +3 bonus to attack and damage rolls made with this magic weapon.  It also deals an extra 1d10 psychic damage to any target it hits.

Sentient Weapon - Somnus is a sentient weapon with an Intelligence score of 14, a Wisdom score of 14, and a Charisma score of 12.  Somnus' personality is curious outside of combat, while cunning and efficient in combat.  When a creature first interacts with Somnus, they must succeed on a DC 18 Wisdom saving throw or Somnus can telepathically communicate with them at will and can probe the user's memories and thoughts.  If a creature succeeds on this Wisdom saving throw, they can choose to communicate with Somnus and their thoughts are shielded from his curiosity. 

Unbound State (requires attunement by a character that possesses psionic traits) - Once attuned, Somnus gains the following additional magical properties:

• An individual wielding Somnus can use either their Strength or spellcasting modifier for purposes of attacks and damage.
• Somnus  also deals an extra 1d10 force damage to any target it hits.
• Somuns also deals an extra 1d10 psychic damage to any target it hits.
• Somnus has a reach of 30 ft.    
• Psionic Resolve - Somnus grants its user a bank of d8 psionic dice equal to their level, which can be added to any Attack or saving throw as a Reaction.  The d8 can be added to the roll after the roll is made, but before it is determined whether succeeds or fails.

The mind is a curious thing when it sleeps, and the curiousness compounds during an eternal dream. 
--Musings, Part IV.  Ilyana Cordenali, 550 Y.K.

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

Notes: Bonus: Magic, Damage: Psychic, Damage: Psychic, Bonus: Melee Reach, Damage, Combat, Versatile, Sap