Auto-translated
Today, I completed this Agrael campaign for the third time, and if you play the thematic mission "The Ship" in the following order, you won't encounter any problems: 1) Take the southern Elven town. 2) Agrael immediately heads northwest (up and to the left) and captures the second town. 3) There's a tavern nearby, and you can hire two more heroes; it doesn't matter who they are – troops only desert from Agrael's army, not from the infernal heroes. Send one to the first town just in case, and leave the other here. 4) Agrael moves east along the road; along the way, you'll find Silanna's tree, and archdevils will join. You'll encounter a couple of weak Elven units. 5) He reaches the eastern town and captures it. This leaves the Forest faction with only one town, Erivell. If you want to play for longer, you can develop it up to green dragons. 6) Now that you've protected yourself from the Elves, as the hero from Erivell doesn't leave the town at all, you can calmly clear the quest demonic town in the basement. There are two ways to get there: either from near the starting point, where there's an entrance underground, or from the eastern town, but you'll have to go through a garrison. That's all there is to it. Agrael finishes at level 26-27, and probably with any skill build. This time, he didn't have Chaos skills at all; somewhere, a witch gave him Summoning, which he doesn't really need, except maybe for Phantom. And Darkness skill.
I should note that if you play version 3.0 of Lords of the Realm, using the corresponding mod to merge campaigns, as I did this time, it's impossible to complete the dragon quest if you have the second upgrade for archers in your army; the scripts don't recognize them. So, I just had to defeat the dragons. And, of course, in Erivell, I recruited demonic troops for Agrael, just for the sake of it... Although you could use elves, but I just don't like them.
I saw on YouTube how someone completed this mission on Hero mode with the starting army, plus the units that join along the way, without capturing any towns except Erivell.