How do I find the hidden units?

Question

There is a help text during the loading screen that talks about "secret units" that can be found during the campaign.

Unfortunately my PC is quite fast and it finished loading before I was able to read the whole text.

Are there really hidden units, apart from the 3 core units and 5 "special" units, and if so, how do I unlock them?

I already played the campaign to the beginning of the Silver Cities and haven't come across anything like that yet.

Answer 1

I suppose this refers to those Units, which you don't get automatically, but need to find in the game world by doing quests for NPCs.

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