Blood Magic

“Perhaps there’s some truth to the idea that the real danger of blood magic isn’t that it draws its power from sacrifice, or that it tempts the greedy and ambitious into using the suffering of others to fuel their spells. Perhaps the danger is simply that we do not understand it, and that lack of understanding invites disaster even when our intentions are pure.” 

―Calien d'Evaliste

 Considered taboo by most societies and magic practitioners, the use of blood magic is a rare art that harnesses the latent powers of a creature's vitality to fuel and amplify the caster's own capabilities, while manipulating and weakening the bodies of enemies from the inside. Some of the more macabre mages seeking to empower their arcane pursuits turn to blood magic as a means of bolstering their spells, giving their own life's blood to reach new heights of frightening magical prowess. 

While blood magic is not inherently evil nor corruptive to it's user, a powerful practitioner of blood magic is often as feared as any lich. Many stories exist of blood mages capturing innocents and using their blood to fuel their spells, or puppeteering an individual's body to perform tasks for them. Blood magic may not be inherently evil nor corruptive, but the types of people who seek to use it are often already going down a dark path...

Note: This subclass is based on Matt Mercer's "Blood Magic" subclass from Critical Role: Tal'dorei Reborn, but has been modified to emphasize the idea of a high-risk, high-reward subclass that can cast powerful spells more frequently at the cost of their own life. Additionally, it has been altered to feel more like blood mages seen in other fantasy works, such as the Dragon Age series. It can be a bit more powerful than the original Blood Magic subclass, which is why it  is recommended that any DM's have the use of blood magic be considered taboo in their societies. It can be a scary power, and should be treated as such by the citizenry.

Restriction

2nd-level Blood Magic feature

Creatures without blood pumping through their veins cannot use blood magic.  As a result, you cannot take this subclass if your creature type is construct or undead.

Blood Channeling

2nd-level Blood Magic feature

When you choose this arcane tradition at 2nd level, you are able to use your own depleted life essence to channel your magical abilities. Whenever your hit points are below your hit point maximum, you can use your body as an arcane focus. In addition, you may use your own life force to cast your spells instead of using a spell slot. Casting a spell in this way does not consume material components, and you take 1d10 necrotic damage for each level of the spell you are casting. This damage can't be reduced in any way. If this damage reduces you to 0 hit points, the spell fails.

Using your life's essence in this way renders the wound resistant to restorative magics. Once you use this ability, you cannot gain any temporary hit points until you are restored to your maximum total hit points.

You may also use the blood of another creature to lessen the requirements of casting a spell. If a creature has been dead for no more than an hour then you may cast a spell as normal, but use the creature's blood instead of any material components the spell would normally require.

Sanguine Burst

2nd-level Blood Magic feature

Also at 2nd level, you learn how to weave your life force into a spell you cast, boosting its intensity at the cost of your vitality. Whenever you roll damage for a spell you've cast of 1st level or higher, you can choose to take necrotic damage equal to the spell's level to reroll a number of damage dice up to your Intelligence modifier (minimum one). This damage can't be reduced in any way, and you must use the new rolls.

Bond of Mutual Suffering

6th-level Blood Magic feature

At 6th level, when a creature you can see hits you with an attack, you can use your reaction to bind your vitality to the attacker and force them to share your pain. The attacker takes damage equal to the damage you took.

This feature cannot be used against constructs or undead. You can use this feature once, and you must finish a short or long rest before you can use it again.

At 14th level, you can use this feature twice between rests.

Crimson Puppetry

6th-level Blood Magic feature

At 6th level, you can manipulate the blood within a creature's veins to freeze them in place.  If you do not already know them, at 6th level you learn the hold person spell, and at 9th level you learn the hold monster spell. These spells are always prepared, and neither counts against the number of spells you can have prepared.  

Additionally, your use of blood magic allows for additional effects with these spells. When you cast either spell using your Blood Channeling feature, you hold them in place not with a paralysis spell, but by controlling the blood within their veins. Each time the creature fails their saving throw, their blood boils and writhes within their body, dealing 1d6 necrotic damage that cannot be reduced in any way.

By controlling their blood in this way you can also force them to act against their will. Once on each of their turns you may move the affected creature up to half their movement speed and have them make a single attack against a creature of your choice. You can have them convey simple verbal messages whether they want to or not, but you cannot use a creature controlled in this way to cast spells.

Any creature you control in this way makes short, jagged movements, and other creatures can immediately tell that the target is being puppeteered, though might not know exactly what is causing it. 

Glyph of Hemorrhaging

10th-level Blood Magic feature

Starting at 10th level, when you damage a creature with a spell, you can choose to curse that creature for 1 minute. While cursed in this way, whenever the creature is hit by an attack, it takes an extra 1d6 necrotic damage. At the end of each creature's turns, it can make a Constitution saving throw against your spell save DC, ending the curse on a success.

This feature cannot be used against creatures that are undead or constructs. Once you use this feature, you can't use it again until you finish a short or long rest.

Thicker Than Water

14th-level Blood Magic feature

Upon reaching 14th level, the blood that flows through your veins is empowered with arcane vigor that mends wounds and helps preserve your life. Whenever a spell or magical effect causes you to regain hit points, you regain an additional number of hit points equal to your proficiency bonus.

In addition, while you are concentrating on a spell, you have resistance to bludgeoning, piercing, and slashing damage from nonmagical attacks.