School of Dark Arts

In great libraries of arch-mages and magical universities, deep in basements, hidden by shelves and sealed behind magical wards are the unspeakable arcana that any proper wizards knows is better off well forgotten. Blood magic and witchings and infernal translations are dangers that hold tantalizing knowledge for those wizards bold enough to wade into the darkness. Some of the wizards of the School of Dark Arts look to master the most dangerous forces to serve as magical wardens. Others look into the shadows and see only a truth that most are too frightened to accept no matter the power it offers. Too often, earnest wizards step forward as the well-intentioned former and find themselves become the terrifying later, twisted by the knowledge they've unearthed.

Variant Rule: Corruption
Dark magic is as dangerous as it is addictive, and over time it will consume you.

You start with 0 Corruption. Whenever you cast a wizard spell of 1st level or higher, you gain 1 point of Corruption per level of the spell cast. If your Corruption ever exceeds 20, you succumb to darkness and become Evil no matter your alignment and are partially under the DM’s control, until such a time as you can reduce your Corruption to 20 or below again. Your Corruption manifests as some sort of noticeable cosmetic feature, such as discoloured eyes, ashen skin, or purple veins, which grow more intense the more points you accrue. You can reduce your Corruption by up to your proficiency modifier each time you short or long rest but no more than three times per day. 

Succumbing to Darkness

Whether you're the DM or a player, maintaining player autonomy is crucial, so dark magic players should work with their DM to work out precisely what it means if they go over their Corruption limit. In some rare and dramatic cases, that can be it for the character (until they show up as a juicy BBEG in campaign 2) but this should only be done after the player and DM have discussed it. In most cases, the dark mage will remain in the scene until the danger has passed (albeit with a new evil personality), then scurry off to do some off-screen nefariousness, the consequences of which they can deal with in character once they've had a nap or two.

Dark Artisit

You are able to accrue power through your eldritch magics. Whenever you deal damage to a creature with a spell, you may gain an equal amount of Dark Energy. When you spend gold to copy a spell into your spell book or use a valuable material component to cast a spell, you may spend your gathered Dark Energy to reduce the required gold cost by an equal amount.

Unsealed Power

Your delve into forbidden arcana is not without reason. Great power lies hidden, waiting for those willing to embrace it. When you cast a spell using a spell slot, you may cast at one level higher than the spell slot you spend. You may use this feature a number of times equal to you intelligence modifier. You regain all uses after completing a long rest.

Terrible Cost

Dark Magic takes a toll on those who would wield its power. As an action, draw on your own soul for power or perform a dark ritual. You may roll a d4 or d6 and lose max HP equal to the number rolled. You regain one expended spell slot of a level equal to or lower than the HP lost. You may use this ability a number of times equal to your intelligence modifier. Your HP maximum resets and you regain all uses after completing a long rest.

Soul Shard

Because of the danger of dark magic, experience practitioners find ways to protect themselves. After finishing a long rest, you may place a shard of your soul into a small trinket of your choice. While the trinket is intact, when you are reduced to 0 HP, you may roll any number of your unspent Hit Dice and recover that much HP. After recovering this way, the trinket then shatters and becomes useless. If you create a soul shard trinket when you already have one, the stored soul migrates, and the first trinket loses its power.

Dark Lord

Your study has granted you a master's understanding of the dark arts.
You gain both of the following abilities:

• Overwhelming Power. When you use a spell slot to cast a spell, you may treat the spell as being cast with a level 6 spell slot instead of the slot used.
• Price of Greatness. As an action you may roll a d8 and lose max HP equal equal to the amount rolled. You regain one expended spell slot of a level equal to or lower than the Hp lost.
  After using one of these abilities, you may not use that particular ability again until you finish a short or long rest.