The Demonologist is a mage class, a pure nuker. In keeping with the Conan worldview of wizards as evil you progress through various horrifically graphic methods of burning or electrocuting people and you can summon a demonic familiar.
It plays very enjoyably although it is mainly based on one button.
Here's a review of the performance at level 10.
There are three options - boost lightning spells, boost fire spells or develop general skills. I opted for fire.
My one feat point give me:
Deeper Fires 1/5 +1.5% damage to Fires of Gehenna (your main nuke)
Fires of Gehenna: 2 sec cast, range 25, 70-81 fire damage single target nuke with half damage splash to nearby targets. Inflicts a lingering Incinerate which stacks to 5 damage per second.
Hellfire Stream: 1 sec cast, 2 sec cooldown, range 20, 36-41 fire damage on target and everything between you and your target. This is better than Fires if you can target a ranged mob and have his melee friends run between you and your target.
Waves of Flame: a cone doing 34-39 damage per second for 5 seconds. This is quite wide and is the best option for multiple mobs that can be expected to stand in it (like 2 mobs running to melee you). On a single target it does less than Fires. Has a 4 second cooldown after it ends.
Demon Familiar: Summons a topless winged lady who helps you out by nuking your targets and boosts max mana of your group. Other spells give 2 cosmetic options.
Protection of Set: This is an hour long buff giving you a damage shield. The shield reduces each hit by 6 but collapses after absorbing 135 hit points. You get a new one every 25 seconds.
Just nuke stuff. It's all very easy this early in the game unless you pull spectacularly badly. You don't even need to optimise spell selection, just spam Fires while watching TV.
That doesn't mean it's not fun. The occasional spectacular fatality where some poor torched dude runs past you screaming makes it all worth while.