Pact of the Blade is a class action available to Warlocks that choose Pact of the Blade at level 3. This is for summoning a new weapon - if you want to bind to an existing weapon use Bind Pact Weapon.
You are Proficient with the weapon. You can only have one at a time. The weapon disappears if you are more than 1.5m away from it for more than 10 rounds.