The Guild of Mages is one of four guilds in Dungeon Lords. When you join it, you'll add the first tier mage class to your character (unless that was the class you selected at character creation). All four guilds work about the same. For each one, you'll have to go on an initiation quest (#1), then you'll be given a second tier class quest (#3-4), then you'll be given a guild quest (#2), and then you'll be given a third tier class quest (#5-6). You can join two guilds in the game, but you can only select one third tier class for your character, so try to plan out your advancement before selecting any quests.
The Guild of Mages has two guildmasters:
1 - Initiation Quest
Deliver the staff of scaris (also called the staff of scorching) to Rianne in the Devil's Marsh.
Rianne hangs out at her shop, and the shop is pretty big, so she should be easy to find. Once you've given Rianne the staff, she'll reward you with a heraldry bonus, your mage class will become active (if it wasn't your starting class), and you'll be allowed to add a second tier mage class for your character.
2 - Second Guild Quest
Go to Skuldoon and help out Feros Duul.
When you reach Duul, he'll tell you that a curse has been placed on him by a rival shaman named Laerok Maa, and he'll ask you to track down Maa in the Forbidden Lands and bring back the idol he has.
You'll find Maa in front of a shack that looks just like Duul's (#2a). Maa will start with some skeletons surrounding him, and he'll summon more skeletons as he fights, but otherwise he's not too tough, and if things start going badly you can always rest at the campfire in front of his shack to regain your hit points and spells.
When Maa dies, he'll drop an idol of death, plus probably a couple of spell books. When you return to Duul, he'll reward you with the level 8 nether spell "summon fiend." If you're playing the Collector's Edition, then you'll also receive 10,000 experience and a point of honor. Then when you return to the Guild of Mages, you'll receive a heraldry bonus plus gain the ability to select a third tier mage class for your character.
3 - Battlemage Class Quest (Class Information)
The outlaw mage Krugas and his enclave of armsmen are stalking the lands south of Lost Lake, killing anyone who ventures there. Find him and kill him.
You'll find Krugas somewhere in the swampy area southeast of the lake. His "armsmen" are skeletons, and he'll have a bunch of them protecting him, but if nothing else they should make him easier to spot. If you have the "fire nova" spell, then one of those will take out the skeletons, allowing you to concentrate on Krugas.
Krugas will cast fire spells, and every so often he'll summon some more skeletons to help him out, but otherwise he's just like every other boss, with a ton of hit points to work your way through. Just quaff potions and cast healing spells as needed to defeat him.
4 - Sorcerer Class Quest (Class Information)
Find the insane sorcerer Mordelus, who is plundering unsuspecting sailors upon the Lost Lake, and kill him.
Mordelus summons skeletons, but otherwise he's a pretty standard spellcaster. We've also heard of him spawning on the thin strip of land west of the lake.
5 - Warlock Class Quest (Class Information)
Find the evil warlock Glavenmaur in the Vale of Ruin, and there defeat him and his battalion of Barrowgrim's soldiers.
When you've defeated Glavenmaur and returned to the Guild of Mages, you'll earn "the raven" heraldry bonus.
6 - Wizard Class Quest (Class Information)
Enter the Vale of Ruin and defeat the wizard Ravensol and his army in battle.
Ravensol starts out with half a dozen demonic bladewitches with him. You won't really need to attack the bladewitches much; just running around in circles will do the job, as Ravensol will kill them for you when he casts his spells. Ravensol also summons skeletons to help him, but otherwise he's a typical spellcaster. For defeating Ravensol, you'll earn "the wizard" heraldry bonus.