During the night you can choose following stackable upgrades:

Upgrade King Attack (max x10)

20
Increases the King's attack damage by 8%.

Upgrade King Regen (max x10)

20
Increases the King's health regen by 0.005% missing health + 0.005% max health

Upgrade King Spell (max x10)

20
Increases your King's AOE ability damage by 9%
Each king starts with one of the following spells:

Chain Frost
Releases an orb of frost that jumps to nearby enemies, dealing Pure damage and slowing attack speed by 15%.

Judgment
Slams his staff on the ground, dealing Pure damage to all nearby enemies

Lightning Hammer
Uses mana to unleash bouncing lightning bolts that deal Pure damage.

Immolation
Deals damage per second to all nearby enemies