Just a minor correction: The appearance of those mounted kings wasn't caused by the summoning of the ermorian bishops, but rather by the randomly generated undead rabble each turn for your ermorian dominion. This is still true in DomII (with the exception of Ermor: Broken Empire, which does not automatically generate units in its provinces).
Please consider it a helpful hint, not a "oh, just someone who has to look like an annoying know-it-all"... that's other people's job.
