"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
You won't be-leaf what he can do!
Leader Ability
Safe Arbor
Allied Minis can be deployed near Onu, and blocks nearby enemy Troop and Leader deploys.
Ancient of Lore, or Ancient of More Minis?
Traits
Vulnerable
Tank
Safe Arbor
Melee
One-Target
Possible Army Slots
Talents
Barkskin
Minis deployed within Safe Arbor gain Armor for 8 seconds.
Petrify
Petrify on death, keeping Safe Arbor active for 8 seconds.
From the Trees!
Minis deployed within Safe Arbor deal double damage on their first attack.