During the night you can choose following stackable upgrades:

Upgrade King Attack (max x10)

20
Increases the King's attack damage by 12%.

Upgrade King Regen (max x10)

20
Increases the King's health regen by 0.0045% missing health + 0.0045% max health

Upgrade King Spell (max x10)

20
Increases your King's AOE ability damage by 6%
Each king starts with the following spells:

Giant Slayer
Attacks deal 1.8-4% max health Pure damage, scaling with 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.