Skills for an Inferno hero.

Um... A Demon without attack? I've always thought, and still think, that attack and logistics are essential for a Demon. Skills and other abilities depend on the situation and personal preference. Machines aren't always relevant at any point in the game, and Demons also have a difficult time leveling up their Fireball skill. Luck is useful for increasing physical damage, as Demons primarily rely on physical attacks. Education isn't strictly necessary, but I like this skill and level it up for almost all my heroes if it becomes available. Of course, you shouldn't rely on Education for a Demon. Dark Magic can replace Luck or be used in addition to it. First and foremost, you should focus on mass spells. If you're lucky with spellcasting and have the time/opportunity to build a guild, you can harass the enemy with Blind and Dominate, or cast Vampirism.

Also, one more thing. If you expect dark magic from the enemy, it's better not to engage in ranged combat, but to immediately attack. Otherwise, by the time your troops reach the enemy, they will already be weakened/slowed down/dominated, etc.