What's wrong with Pyromancy? Bad? Good? I want to play as one when this comes out.
It's a form of spellcasting that is completely independent of stats. Instead of relying on the character's stats to increase the power you spend souls at one of several trainers to increase the level of your Pyromancy Flame, which increases the power of all Pyromancies you cast with it. At maximum level a Pyromancy Flame can't deal as much damage with a spell as can an Intelligence build with Sorcery, but it comes reasonably close and the Sorcery-using character needs to invest 35-40 levels of stats into Intelligence to reach that level of power. The Pyromancer, by contrast, has to invest absolutely no levels whatsoever, only the 300,000-odd souls necessary to fully upgrade the Flame. Not chump change by any stretch of the imagination, but really not all that huge of an expenditure in the grand scheme of things.
The end result of all this is that it's a big component of griefing builds. You can't invade players below your level outside of a handful of specific scenarios, so if you want to hunt newbies you have to keep your level down. So instead of investing souls in levels you spend them all on upgrading your weapons and Pyromancy Flame, allowing you to hit well above your weight class, so to speak. Then you start invading early-game areas loaded up with your maxed-out Pyromancy and non-scaling weapons. Properly kitted-out your low-level griefer is next to impossible to kill using early-game equipment and a single hit from any of the griefer's weapons will cause a low-level, poorly-equipped character to explode like a meat balloon.
The truly elite griefers can exploit the game's naive confidence in individual client machines to kill their victims before they've even spawned in the victim's world. From the point of view of the invasion victim their character just suddenly bursts into flame, falls dead, then watches their murderer appear out of thin air and Praise the Sun as they keel over. It's actually kind of impressively creative, albeit not very nice at all.