Strengthened Bond

Sword of the Sixteen Wishers, Pact of the Blade feature

You feel that as you accepted the sword given to you by Satan, your bond with your patron was strengthened. Three times per long rest and when you hit a creature with your pact weapon, you can choose to deal an extra 2d8 force damage to the target.   You feel as though your bond with your patron could be strengthened further, but how you do not yet know. You may be able to discover this information through research.