The griffin is immortal.

The second mission is a real nightmare, and not just because of its difficulty. I couldn't complete it with my hero. I tried on Veteran difficulty. Victor arrived on the 4th day both times, and, by the way, the fire warriors didn't reach level 5 either. But that's just the beginning. I can't seem to finish the mission. The problem is this: I couldn't build the Angels for a long time because I couldn't gather enough troops to defend the mine; there were too few resources and money. Later, it wasn't critical to keep running back and forth to defend it (I should have been busy chasing down the demons), so I eventually gave up on that. When I built up a decent army and managed to take the garrison, I defeated the blue player. This probably backfired on me. While I skipped weeks, letting Markel gather his strength, I went to take the gem mine, and to my surprise, it turned out to be a quest-related mine and a very strong one. This quest appears in the main storyline, but since it's already completed, it can't be closed. The situation is this: Markel defeats Aiden, it seems, but he can't reach the garrison – the mission fails. Fiona can defeat the first Inferno Castle, but the path to the second is blocked. I even tried using a town portal to reach Markel and attack from there – the mission still fails (when Fiona approaches the garrison, but otherwise they communicate and exchange artifacts). I tried not touching the mine at all – the same dead end. Conclusion: the map is poorly designed; it doesn't account for perfectly legitimate player actions that make it impossible to complete. People have already written about Durgal – he's useless (they could have kept him in the dungeon and leveled him up, or given him a Tome of Darkness/Light, or at least a useful scroll), especially since he's freed at a late stage in the game. It would be good to display the tasks of the keymasters and farmers in the quest log – you don't always play continuously, and then it's hard to remember what they asked for. After Markel's dialogue with Aiden, it would be good to show him on the map (open him on the map) – they did meet, and also display Markel's decision in the quest log. It's difficult to build up an army for Markel – otherwise, it's impossible to defeat Aiden (although I didn't have the patience to reach the number of creatures shown in the screenshots above). Unfortunately, my playthrough ended here.

Added 22 minutes ago
And I'd like to elaborate on this part. Fiona starts the mission at level 1, with light, darkness, and a fallen knight.