The game, then, is a dungeon hack with a Doom-like sheen; London has been perverted and damaged by the demonic invasion, and you'll explore and quest your way through it while raising your character, seeking powerful equipment and learning new arcane abilities. The Cabalist isn't just about magical projectiles, for instance. These characters can also take on demonic attributes by using special potions brewed from demon body parts - changing a hand into a claw, for example, and then using it to wreck monsters. If you're turned off by that idea, you can decide to summon your own demons and use them against their comrades too.
The dungeons are randomly designed, and in fact, what lays between two points on your map may change entirely - you'll take a route through a sewer one time, and caverns the next. You can warp back to a safe spot any time you want, so you can build your character as much as you like before venturing safely onward. If you choose to move online and adventure with others, Hellgate, being a more controlled experience, is not like MMOs where every area is crawling with players. Each dungeon is a private experience for a chosen few players - known as an instance. You can lock out unwanted players so you never have to wait for others to get done before starting your adventure. You can also let people in and out as the need arises, so you're not screwed if a couple of your friends lose their connection to the net.