In an experiment that should've never happened, the Weapon of your pairing has been melted into black blood which was then used to replace the blood of the Meister, fusing both of your souls. This results in perfect soul resonance being incredibly easy to achieve for the two of you, as well as make both of you a lot more resilient than before against physical damage. It also gives the Weapon incredible flexibility, as they control the black blood directly. However, this can also lead to unstability, as souls aren't meant to be fused and the Weapon is now fully dependent on the Meister, making it harder for the two of you to act as equals.
If picked alongside Pas de Trois, one or more of the Weapons in your team are merged in this way with all the other members of the team, Meisters and Weapons both.