During the night you can choose following stackable upgrades:

Upgrade King Attack (max x10)

20
Increases the King's attack damage by 7%.

Upgrade King Regen (max x10)

20
Increases the King's health regen by 0.006% missing health + 0.006% max health

Upgrade King Spell (max x10)

20
Increases your King's AOE ability damage by 7%
Each king starts with the following spells:

Giant Slayer
Attacks deal 12-20% current health Pure damage depending on Upgrade King Attack

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

Tenacity
Gains 1.3% damage reduction for every 1% damage taken this wave.