Prerequisite: Pact of the Blade
You can use any weapon you summon with your Pact of the Blade feature as a spellcasting focus for your warlock spells.
In addition, the weapon gains a +1 bonus to its attack and damage rolls, unless it is a magic weapon that already has a bonus to those rolls.
Finally, the weapon you conjure can be a shortbow, longbow, light crossbow, or heavy crossbow.
Source: XGE, page 57