Build:Sigil Mage

Full Build Details
This build generally requires a lot of prep time for it to output it's full damage potential. You want to start by using all 5 elemental boon skills (Warm (skill), Cool (skill), Blessed (skill), Mist (skill), Possessed (skill)), or their respective potions if you have yet to acquire them. Rage helps immensely, as most of your spell combos will do Impact damage. Discipline is optional, as it only effects part of one skill combo, but if you have the time to prep, you might as well throw it on (more useful if you use a different weapon type than a staff).

Once all your Boons are active, it's time to put down all your Sigils, starting with Sigil of Wind, as it takes the longest to cast. Keep in mind, using the Sigil of Wind requires you to have found the wind altar in the region, so it's effectively your go to "free" cast when you don't want to use/waste one of your mana stones. With your boons active, you should be at, or close to, 100 resists in fire, frost, and decay, granting immunity to those damage types, as well as damage from the Burning and Poisoned conditions. Then, start casting your spells, and watch the enemies melt!

We go Heroic Kingdom for Alchemical Experiment, as it's a 15% damage boost to everything (which we do everything!). You could also go Divine Assistance from The Holy Mission of Elatt quest-line if you plan on using an alternative gear set with little mana cost reduction, picking up a 20% decay resistance as well, potentially allowing for a gear swap while still maintaining the decay immunity.

My preferred base mana with this gear set is 40 or 60 (2 or 3 sacrifices), but you can pick up more to help you through the early game if necessary (don't go over 100), or if you plan on having less mana cost reduction by using an alternative gear set.

Mana Stones
This build requires the use of three separate types of Mana Stones; the Fire Stone to cast Sigil of Fire, the Cold Stone to cast Sigil of Ice, and the Dark Stone to cast Blood Sigil. Fire and Cold Stones are relatively easy to find off enemies or make, requiring only a Mana Stone and either Thick Oil or Blue Sand, respectively. Dark Stones are a bit more rare, with fewer enemies dropping them (only from the Antique Plateau region AFAIK), and even harder to make - 50% Corruption with a Mana Stone and the Cleanse skill. Thankfully, with this build, the decay damage done by Cleanse should be completely mitigated by your decay resist, so you only need Bandages to remove the effect of Bleeding. Otherwise, it's just a matter of finding a corrupted area and sitting in it to gain corruption (and therefore Dark Stones), or power eating foods that grant corruption (only recommend this if you have no corrupt resist, or a boatload of silver to spend). Keep in mind that the use of skill combos on your Blood Sigil will also give you corruption, so you'll often find yourself able to make another Dark Stone after using one or two.

Enchantments and Gear Alternatives
The reason why we go Ivory Master's Staff is for the Isolated Rumination enchant, granting us a much needed Cooldown Reduction on our skills, allowing us to keep using our spells in fights, and go from one fight to the next with minimal downtime. Some alternatives here, the Rotwood Staff gives a good damage bonus and an extra 5% mana cost reduction, the Thermal Claymore also gives some good damage bonuses if you prefer to have an actual weapon in combat and don't mind the mana cost reduction loss.

You can replace the White Priest Boots with Jade-Lich Boots for more decay damage while keeping the decay immunity (important for Cleanse), and an extra 5% mana cost reduction (especially important if you don't go with a staff). Tenebrous Boots can be a decent choice if you want more damage, and don't mind losing the elemental immunity on fire, frost, and decay damage (will only have ~90% resists). Crimson Plate Boots are also an okay choice if you can get them from a Blue Chamber playthrough, but you lose the mana cost reduction and fire/decay immunity. Gold-Lich Boots are also worth considering here as they can give you a much-needed boost to your often negative lightning resist from being Tainted. Flux is an amazing enchant for any pair of boots on this build.

We go White Arcane Robe with Stabilizing Forces for the three elements immunity and some impact resist, but a good replacement is Jade-Lich Robes, as it allows you to keep the decay immunity and add more damage, only losing the fire and frost immunity. Crimson Plate Armor gives you more frost damage and physical protection at the expense of mana cost reduction (important if you don't have a staff) and the fire/decay immunity, but requires yet another slot in a Legacy Chests. Antique Plate Garb can be considered for this slot if you can get it's unique enchant (Spirit of Harmattan) and can afford the mana cost reduction loss/don't mind losing fire, frost, and decay immunity for a hefty damage buff and some cooldown reduction.

The White Priest Mitre is insane for this build, and should probably not be replaced. Enchant with Elatt's Sanctity (preferred) or Aegis.

An alternative gear set is the Rust Lich Set, it grants insane all-round damage (25% to all elements except ethereal) at the cost of physical damage (not super important if you have a staff or elemental only weapon). You lose all resists from your gear, so you would only have what your boons give you. Think of this set as a "glass-canon" option. Recommended to replace the helmet in this set with the White Priest Mitre for more damage and resists (you shouldn't need the 15% mana cost reduction). Keep in mind that using a Mitre with an enchanted Antique Plate Garb and Tenebrous Boots will give you a similar damage output to the Rust Lich Set with more resists, and without the physical damage reduction. However this gear set has very little mana cost reduction unless you use a staff.

An alternative backpack to the Light Mender's Backpack is Mefino's Trade Backpack, as carrying around all those mana stones can be cumbersome on your carrying capacity, but keep in mind you will lose some lightning damage.

Combos
''Stand on the sigils at the start of combat to gain a very potent buff to four different damage types, at the cost of losing health per second. Your fire resistance should mitigate the burning from Immolate, but you'll still take quite a bit of health loss from Gift of Blood, so make sure you have some Health Recovery going. As an added benefit, you can cast this again mid-combat to either heal an ally, prevent a lethal attack, or just do some AoE frost damage around you on the sigils.''

If you have the time to cast this from your inventory after using Mana Ward on your sigils, it's a nice source of extra damage, but don't feel pressured to get this down.

''Stand on the sigils to fire a long range projectile that does ethereal and frost damage, as well as decay and physical damage if the enemy is close. Has high impact damage.''

''Stand on the sigils to cast both a decent ranged fire projectile and close ranged lightning blast. Spammable.''

''Stand inside the sigils to create a turret that does decay damage per second to a nearby enemy. Will also summon a very high damage lightning bolt AoE from the sky. As an added benefit, if you are forced to leave the sigils, you can recast this and it will still call down the Lightning Strike, hitting all enemies on the sigil.''