"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.
Summary
Firelord, Lord of Flame, Master of all Elementals, you get the idea.
Leader Ability
Molten Armor
While in play,
Even without legs, you can still have a Molten Core!
Traits
Elemental
Resistant
Tank
Braze
Blast Wave
Melee
AoE
Possible Army Slots
Talents
Concussive Blast
Blast Wave leaves enemies stunned for 2 seconds.
Radiant Flames
Buildings you control periodically damage nearby enemies.
Sons of Flame
At 50% health, summon two Fire Elementals that last 8 seconds.