It was a waste of time to replay it. I did level up my mentor, and the Dark Elf got to level 25 for free from level 4. But he was only useful once, when he was forcibly summoned for a battle before the finale. Before that, he was just tagging along – I could have just put him in the Castle and forgotten about him, but I still had to send troops to that battle.
As for what's in the 5th game, the main character can't access the underground area, no matter what the Dark Elf does. You can only go into the underground from within it, but not the other way around – only mages can go there.
The mage garrisons are physically inaccessible; it's not just that the danger is high, but the hero simply can't go there, only mages can pass through.
But since the main character can fly, I flew down and easily defeated everyone one by one with my 50 attack points and Dark Elves, then returned to take back the cities. And that wasn't even necessary. There were 2 cities that I wasn't using, and a couple of stats. If the hero couldn't fly through portals, I would have had to defeat everything from scratch with the Elf, but he would have just stood there guarding the cities, as the entrances and garrisons are inaccessible.
Overall, I liked it, except for two things.
The Grail quests are very poorly designed. In the 5th mission, I collected the obelisks, but I couldn't dig up the Grail. Because I got tired of looking for it. The identifying features were a mountain, three pillars, and another pillar. But I couldn't find anything like that. I spent about 15 minutes searching and gave up. Some people think that if they force the player to skip turns, walk around, and examine the map millimeter by millimeter, the game will become more interesting, but in reality, it's not very enjoyable.
If the Grail is set by the author, they should put it somewhere with more identifying features.
There's a similar problem in the original campaign for Rai'lag. Usually, everything is fine, but once, the Grail was placed in such a way that after half an hour of frustration, I had to use a cheat to win. The Grail map in Heroes 5 is just bad. In Heroes 4, even the troops and hero are displayed, and in Heroes 3, at least the entire area is fully displayed, even if without armies. In Heroes 5, the Grail map is just garbage. The same problem exists in the 2nd mission with the bugged obelisk, forcing the player to dig around endlessly when the Grail map is already bad. And in my case, the Grail was also placed somewhere where it was impossible to find, even though I was getting messages that it was to the left or right.
Also, it's probably necessary to consider the mandatory need for specific skills. For example, in mission 1, I focused on machines, logistics, and attack. Neither of the first two was needed in the second mission, and there was simply no way to get machines. The map itself isn't very large, but logistics are usually useful. The lack of defense made the second part very difficult, as it was hard to defeat hordes of fast and/or ranged creatures with a handful of low-level units. And the lost item slot for summoning also makes itself felt, reducing the number of skill slots up to the mentor (end of chapter 3). Or you need to know in advance that there won't be machines and there will be many opponents, so level up carefully. Or you have to replay the game. Or, if you don't take the necessary skills, you have to waste time in battles until you suffer adequate losses. At the same time, you need to keep in mind that the 3rd chapter will be very difficult to pass without logistics and machines.