Demon invasion.

I should note that I didn't encounter either a Markal or an Arantir throughout the entire campaign; are they prohibited?

Well, generally, when merging maps into a campaign, I prohibit all heroes from the campaigns from appearing on them, but I must have missed someone.

Added 1 minute ago

which, in theory, is only in the library under strict guard

The spells themselves are not prohibited on the map. In the main castle of the Humans, you can build a guild up to level 5; you can also use an eagle to learn spells from Vittorio if he is willing to use dark magic...