Creative Assembly was very circumspect when it came to the details of how these are actually triggered in the campaign, but the answer from Roxburgh so far is that these are 'kind of boss battles at the end of seminal gameplay moments within the realms - they are you completing that section, that requirement, in order to progress with the storyline.' There are just four of these in the game's campaign, one for each Chaos boss that you'll need to defeat if playing as one of the typical 'good guy' factions like those two new ones Kislev and Grand Cathay.
