Auto-translated
Demons are most likely to acquire Logistics and Offense skills (15%). Battle Machines, Unique skills, and Chaos Magic have a slightly lower chance (10%). Luck, Defense, Sorcery, and Dark Magic have an 8% chance of being added to the hero's skill pool. Very rarely acquired skills include Education, Leadership, Magic of Light, and Summoning (2%). However, your task is not to choose the most frequently acquired skills, but the most useful and effective ones specifically for Demons.
As you know, the greatest strengths of Inferno are Hell Gates and Hellfire. Based on this, your hero needs to develop skills that enhance the effect of Hellfire and increase the number of units and reduce the time it takes for units summoned to the battlefield through Hell Gates to appear. Demons have three skills that can potentially enhance the Hell Gates ability, and two more that enhance the effect of Hellfire (in fact, there are more of these skills – I'm just talking about the ones that will be most useful to you and that your hero can easily upgrade).
Logistics, Gate Opening, and Luck are skills that can improve Hell Gates during the game.
Let's start with Logistics, which is simply essential for the Demon Lord. If you look at the Skill Wheel (I assume you've already downloaded it, but if not, don't worry – I'll explain everything in detail), you'll see that this skill unlocks two very important abilities: Quick Gate Opening and Sudden Attack. The first ability reduces the time required for reinforcements to appear from Hell Gates by 50%, and the second gives the hero the ability to teleport any unit to any point on the battlefield, and the initiative of the teleported unit increases so much that, most likely, the teleported unit will move after the hero's turn. And logistics itself is a very good and useful skill. Luck can help you increase the number of units from Hell Gates. Your hero will have the opportunity to acquire the Wide Hell Gates ability. This ability allows all your units to summon twice as many creatures from Hell Gates as usual, with a 15-30% chance.
Gate Opening is a unique skill for the Demon Lord. At the beginning of the game, your units can only summon 25% of their number from Hell Gates. However, if we upgrade Gate Opening to the fourth and final level, the size of the summoned units will be 40% of the summoning units. Chaos Magic and Defense can help you improve Hellfire.
Withering Flame is a Chaos Magic ability that increases the damage from the use of Hellfire by 50%. In addition, Chaos Magic enhances the Fireball spell. It can be used not only by the hero but also by Cave Demons and Lords. This is very important! When you upgrade Cave Demons to Lords, they will have a new spell – Meteor Shower. Here, we are somewhat limited – the Lords will only have enough mana for one Meteor Shower or two Fireballs. The choice is yours. Fiery Retribution allows the units of your hero to also inflict Hellfire damage on the enemy during counterattacks.
To make it easier for you, I present a linear development for the hero for each skill:
1. Logistics basics -> Pathfinding -> Advanced Logistics -> Quick Gate Opening -> Master Logistics -> Soul Eater -> Sudden Attack.
2. Ghostly Luck -> Soldier's Luck -> Great Luck -> Wide Hell Gates.
3. Defense basics -> Dodge -> Advanced Defense -> Hellfire -> Fiery Retribution.
4. Chaos Magic basics -> Fire Lord -> Strong Chaos Magic -> Hellfire -> Withering Flame.
5. Gate Opening basics -> Hellfire -> Soul Eater -> Advanced Gate Opening -> Master Gate Opening -> Absolute Gate Opening.
6. Machine Management basics -> Ballista -> Advanced Machine Management -> Master Machine Management.
The last, sixth skill will change depending on which hero you choose (in this case, let's assume you chose Daleb and the sixth skill turned out to be occupied by Battle Machines). I doubt you will be able to upgrade your hero to this level, but that's not the point – your main goal should be to defeat the enemy, not to upgrade all the useful skills