between two units grants them a unique "Bond Combat Art." For example, an A-Support between Sylvain and Annette might unlock a unique rally or a dual-strike maneuver.
Make sure you have the latest firmware (at least 12.0.0) and Sigpatches. The DLC unlocks correctly if you load via the main story – talk to the mysterious man in the Abyss entrance after Chapter 4.
Set in a secret underground society called "Abyss," you control a fixed party of Ashen Wolves. It is a linear, challenge-focused dungeon crawl that plays like classic Fire Emblem (no monastery downtime).