The Wizard Guild in Erudin is called the Crimson Hands and can be found on the upper floors of the Erudin Palace. On the top floor of the Hall of Crimson Hands you can find the Wizard Guildmaster Ghanlin Skyphire. The Crimson Hands Guild lend their magical support to the High Council and High Guard of Erudin, and oppose the Heritics of Paineel.
There's not a single quest I know of to raise Crimson Hands without lowering Heretics. But you can do two things to achieve the same result.
Do whatever quest raises Crimson Hands and lowers Heretics. Then kill kobolds in the Warrens to raise Heretics.
If you're high-level, you can mass-murder kobolds really fast for all the Heretics faction you want. And you can kill skelly guards in Paineel for +Crimson Hands / -Heretics once Heretics faction is high, returning to Warrens occasionally to stay friends with the Heretics even though you're terrorizing the city guards...hehe.
The skelly guards in town seem to be around level 15 and you can single them out without engaging harder mobs in there. DON'T tackle the skelly guard in the newbie area: he's not like the rest! He cons the same but he has a bazillion hitpoints and I thik he hits harder.
Bonus: they are guaranteed to drop a bronze sword and a kite shield, netting around 8 plat a kill. You can farm them for decent money considering they're level 15.