Spellcasting. Zeus's spellcasting ability is  Wisdom, spell save DC 20, +10 to hit with spell attacks. Zeus can  cast following spells: 

At Will: Lighting Bolt, Chain Lightning, Thunder Wave

Legendary Resistance (3/day). If Zeus fails a saving throw, he can choose to succeed instead.

Thunderstorm. When in battle, Zeus summons a large cloud that surrounds the arena and deals damage to any PCs in the arena. This deals 2d10 Thunder Damage at the end of every round.

Storm Master. Zeus is Resistant to Thunder and Lightning damage.

Power Surge. All of Zeus's melee attacks deal an additional 4d4 Lightning damage.

Multiattack. Zeus can either do 3 unarmed attacks, 2 melee attacks, or 1 magic and 1 melee attack.

Unarmed. Melee Weapon Attack: +5 to hit, reach 5 ft., 1 target. Hit: 10 (2d4 + 2) Bludgeon damage. 

Blade of the Storm. Melee Weapon Attack: +10 to hit, reach 5 ft., 1 target. Hit: 30 (4d6 + 6) Piercing damage.

Storm Strike. Every 4 rounds, the Thunderstorm surrounding the arena casts Lightning Bolt on a random enemy in the arena.

Zeus can use 1 of 3 legendary actions at the end of his turn, but must wait before he can use another.

Gale Force (Costs 1 Action). Zeus launches a blast of wind at an enemy and forces the enemy back. The enemy must make a DC 15 Dexterity Save or be pushed back 100 feet on a failed save, half as much on a successful one.

Thunder Clap (Costs 2 Actions). Zeus claps his hands together and all enemies within 100 feet take 16 (6d4) Thunder damage. 

Call of Olympus (Costs 3 Actions). Zeus lets out a shout that sounds like pure thunder, and summons 1 godly artifact of his choice and uses it on his enemies for 1 attack.