Relay Defense is a defunct game mode in Fire Emblem Heroes.
Player assemble a brigade to defend forts on a map while simultaneously defeating strong enemy Heroes. Each attempt consumes 10 Stamina and as the player completes a map, higher difficulties including stronger enemy units with higher stats and better skills emerge. The first five units in the player's brigade are active on a map upon starting the game mode. Players can use their heroes to defend their three forts or attack enemies. The player gains a score based on how many enemies are defeated or how many camps remain as well as their Fort's health. In addition, the player is scored based on how many of their brigade units survive. Every two full rotations of player and enemy turns, all of the player's current active units deployed are despawned from the map and are counted towards a survival score. After they are accounted for, the next five heroes in the player's selected brigade appear on the map at the designated summoning platforms.
The game mode only occurred twice on September 18, 2018 and October 18, 2018. Both events lasted a week.