Skills for an Inferno hero.

I don't see any point in developing Light magic at all: firstly, there are no Light spells in the guild, and secondly, isn't the general attack speed boost enough for you? In my opinion, Darkness is the best for Demons!!! Mass haste, then something else (forgetfulness, slow), and it's just great))...

It has already been mentioned here that Divine magic is perfect for Demons, and that's true. And if it happens that the guild gives you Divine magic, a Light spell drops, and you also have a couple of other spells from that school, then you can consider being a Light Demon. Or do it in the campaign for Agrael, where there are plenty of Elven towns, and therefore, plenty of spells.

Darkness is good for a Demon, no doubt, but it can drop quite late. Sometimes I don't develop magic at all for a Demon, I just rely on a surprise attack and mass haste from the attack, and I try to get a ballista, almost not using gating. As a result, at fairly low levels, I get a quite powerful and fast attack. (by the way, instead of surprise attack, you can also take sharp mind, especially if we are developing Darkness; make mass confusion/slow/haste/weakness the first custom spell – it's very effective)