Whether Xerces should be included in an array of gods rather comes down to how one defines god. In this case, divinity is defined as “can do anything he likes, regardless of the sensibleness or otherwise”
Historically, he’s your basic emperor. Mythologically, he’s a megalomaniac with supernatural capabilities at the least. Calling your elite troops Immortals, and your bureaucracy a priesthood will sow the idea that you think of yourself as divine. We can’t be sure how he thought of himself; only the propaganda he allowed to be circulated about him. There are suggestion that he may in fact have been a Zoroastrian.
Story – the Hot Gates
The Persians sent an envoy to Sparta. “If we conquer your province, your gold will be ours, your cattle slaughtered for our feasting, and your women will fill the hills with their lamentations”
The spartans returned a single word
Because of the boldness of the Spartan land army, and the strength of the Athenian Navy, Xerces was counselled by his advisors not to invade Greece. Because he was headstrong, and considered his armies invincible, he invaded anyway.
Sparta was far from the front lines, and most of the Persian threat fell upon its ancient rival Athens. Sparta’s damos, its council of citizens, voted not to fight.
King Leonidas saw further than his fellows, and knew that sooner or later, the Persians would need to be fought. It was better to fight with allies on ground of his choosing, than alone before his city.So he went to fight with his 300 personal guard. And they marched to Thermopylae, which was a pinch in the mountains. The Spartans and their allies need to hold the Hot Gate mountain pass for long enough for the Athenian navy to come up with reinforcements.
Two days passed and the thousands of troops, including the Elite Immortals were blocked there.
On the third night, seeking the wealth the Persians so vicariously displayed Ephialtes betrayed the Greeks, and showed the Persians a hidden route through the mountains.
With troops both in front and behind him, Leonidas and his 300 warriors could not hold, and all were killed. The death of their king goaded the Spartans into war with Persia, and they began their march.
With little to impede them, the Persians overran Greece, and burned Athens to the ground. Once the Greeks had united their scattered forces, their combined might pushed back the Persians back to the Hellespont, and the straight to Asia.
Shortly after, the assassination of Xerces diminished the Persian threat.
In your Games and Stories
Xerces is a powerful human but not truly a god – so he can’t directly exert divine power or grant spells. However, he and his bureaucracy are experts in the more mundane ways of making it appear that divinity is at work – why do you need Befriend if anyone in power is ordered to treat you with respect? Can fireballs be substituted with weapons of war? Remember here Arthur C Clarke’s Law of Technological Superiority – any sufficiently advanced technology is indistinguishable from magic. High priests might in fact be studied wizards – or engineers
True followers of Xerces are likely restricted to the most fanatically nationalistic Persians. There may be many knees forcibly bent, but few who truly believe. Therefore, anyone who is outside the Empire likely has some ulterior motive in declaring themselves a devotee of the God Emperor.
Within the Empire, public holidays dedicated to the Emperor are likely to disrupt travellers’ intentions. If you need to buy food, but today is a fast day; if you don’t fall to the ground as the statue of the Emperor goes by – you’re likely to upset the locals. They may even decide to denounce you to the priesthood as a heretic…