You are both an accomplished warrior,  and experienced caster. From using your martial and magical abilities in tandem with your chosen attuned weapon you have honed your abilities to work in conjunction. 

• Your attuned magical weapon can be used as your spell casting focus, while also being able to fulfill the somatic component of your spells when it is equipped. 
• You can chose to use your spell modifier in place of your STR or DEX modifier when making an attack with your attuned magical weapon.
• You gain advantage on CON saving throws against breaking your concentration when equipped with this weapon.