"Army slots" are the bottom 3 slots in each build. They are awarded from successful dungeon completions and every leader has a distinct pool of options. Each slot can be changed after unlocking by spending Arclight Energy.
Bane of the Scarlet Crusade
Army of the Dead
Rivendare periodically summons Skeletons at buildings you control.
Periodically sacrifice a nearby Skeleton to be Healed.
Nearby allied Skeletons gain Bloodlust.
Chill Of The Grave
Summon Skeletal Mages instead of Warriors.