Learn Farsi: ای پادشاه، سپاه اهریمن رسیده است.

The particle ey translates to O (like O king). It is a formal or literary vocative marker used for direct address. While in everyday conversation you might just say a person's name or title to get their attention, adding ey elevates the tone, making it perfect for historical, poetic, or highly respectful contexts like speaking to a monarch.

Since we are learning about both ey and the suffix -â, could we say pâdeshâhâ here instead?

Yes! You could say pâdeshâhâ using the literary suffix, or even combine them as ey pâdeshâhâ. All of these are valid, formal ways to address a ruler. Using ey pâdeshâh is very common in historical storytelling, while the -â suffix feels slightly more archaic and poetic.

I've seen other words for demon like div. Does ahriman have a specific nuance here?

Yes, ahriman carries heavy cultural weight. It originates from Zoroastrianism as the proper name for the destructive spirit or ultimate evil (similar to the concept of the Devil). In epic literature like the Shahnameh, it is used to denote a supreme demon or the absolute personification of evil. It sounds much more epic and threatening than a common monster.

Why does the sentence use the present perfect resideh ast rather than the simple past resid?

In Farsi, the present perfect is frequently used to deliver news or announce an event that recently happened and has immediate, urgent consequences for the present. The demon's army is here right now, so emphasizes the current state of emergency much better than the simple past would.