Alliance boss battles are campaign boss battles with higher difficulty settings. Their difficulty level was designed to withstand the might and power of an entire alliance.
Alliance members can join the battle and send troops to attack the summoned boss. This will increase the chances of looting greater rewards at the end of the battle. The higher the difficulty of the boss, the better the drops.
Where can you find an Alliance Boss?
Alliance Bosses spawn randomly while a player is attacking and winning on the Campaign Map. Anyone can summon an Alliance Boss, as long as that Boss isn't active.