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The problem lies specifically with HD+. This option, in addition to replacing the terrain of battlefields, also affects other gameplay aspects. With HD+ enabled, that hero was killed during an event with the Nagas. Fortunately, in Xiedu, this happens in plain view of the player, and a quest giver nearby demands victory specifically over that hero, so it was obvious that something was wrong.
However, when playing Les prémices de l'Apocalypse, I was less fortunate; the key hero died in a similar event, but I only realized this at the end of the playthrough. But even without that, it's a rather confusing map, although not uninteresting.
If I remember correctly, the map is French, and around the 4th or 5th month, legions of heroes arrive, which signifies the apocalypse.
During my first playthrough, when I saw this, I immediately quit and started over. By firing and hiring heroes on the same day, I was able to speedrun the map significantly. As a result, I arrived at the computer's location before the 4th month and captured all the heroes with legionary armies, i.e., I took them for myself. I liked the map, even very much.
Added 6 minutes ago
Back then, I was playing without SoD_SP, so I wasn't aware.
Added 4 minutes ago
The core of Les prémices de l'Apocalypse is based on several interesting original ideas (the inability to capture the key hero for the scenario at the beginning of the map, the need for active interaction with an ally, playing through fairy dragons), but overall, the map gives the impression of being unfinished. Even if the hero La Discorde doesn't die in the event, it will soon be easy to capture him.
The map is very interesting, and it can be completed in different ways. The main thing is to do everything quickly. La Discorde can be captured, at least, I managed to do it at the beginning of the second month, but there is no benefit from killing him (because, as far as I know, the map is won immediately after that). Therefore, I had to open the pillars for this hero myself, then "scare" him, forcing him to jump into the water and move him to specific points on the sea so that he would open the guardians for my further progress.