The Power of Love is a sword Scott Pilgrim unlocks in volume 4 after telling Ramona Flowers he loves her and levels up. During this same level up, Scott also received Guts +2, Heart +3, Smarts +1, and Will +1. The sword counts as a longsword, for which Scott chose as a proficiency back in grade 5 over the skateboard proficiency.
In Scott Pilgrim's Finest Hour, Gideon steals the Power of Love off Scott causing him to level down, then impales both him, Ramona, and destroys Ramona's Subspace Suitcase with it. However, when the suitcase explodes, it's due to Ramona blocking Gideon's attack, thus disarming him. Ramona then takes up the Power of Love (having been healed by acquiring it), and she and Scott destroy Gideon together.