The Barbarian King is basically a larger, more powerful Barbarian. He is automatically summoned once the Barbarian King Altar is constructed, which costs 10,000 Dark Elixir.
The Barbarian King is an immortal unit, so he only has to be summoned once. However, if he is injured or falls in battle, he must regenerate his health by sleeping for a period of time before he can be used again. If you attack a village while its Barbarian King is still regenerating, the Altar will be empty.
The amount of time he sleeps to regenerate is directly proportional to the health he has lost by the end of the battle. If he is not damaged (or is fully healed before the battle ends), then he is ready to fight immediately; however, if he falls in battle or is wounded, then he must sleep for some time to regenerate.
The more the Barbarian King is upgraded, the more time it will take to fully regenerate his health.
The Barbarian King sleeps during the time it takes to upgrade along with the normal costs (a free Builder, Dark Elixir and time).
You can temporarily quadruple the hero’s healing rate for 1 hour by spending 5 Gems.
The Barbarian King now only defends his territory around his platform and will retreat if lured too far out.
Starting at level 5, the Barbarian King obtains an ability called Iron Fist which can be used on the battlefield. It partially heals him, summons up to twenty additional Barbarians and grants the Barbarian King and any surrounding Barbarians increased damage and speed. This does not stack with the Rage Spell, and does not affect any other troops except the Barbarian King and nearby (friendly) Barbarians.