Base Class: Barbarian
This unrelenting sickness seems to manifest itself in unexpected and nearly unbelievable bouts of compassion also. They are fiercely defensive of their friends and comrades and will throw themselves into harms way to prevent others from knowing pain which so rightly belongs to themselves. As dedicated students of pain and suffering Flagellants can be adept at healing or perform profoundly effective unconventional treatment of wounds. One might expect that this level of caring would exist in opposition to their savage and aggressive nature. The only explanation I could fathom for this is they must at least understand that the dead can't feel pain. To protect the life of a friend, is to ensure that friend can suffer in the future.
Exsanguination
At third level the Flagellant becomes specialized in small 1 handed weapons. When attacking, wielding a Club, Dagger, Light Hammer, Sickle or Whip in 1 hand and no other weapons or shields, the Flagellant can add their Constitution modifier to the attack roll. While under the effects of Rage, the Flagellant may also add their Constitution modifier to the damage roll.
While wielding one of the above weapons, after taking the Attack action on their turn, the Flagellant may as a Bonus Action make an Attack (Melee or Thrown) against a creature they have not damaged with an attack this Turn. With this attack the Flagellant harms themselves, hit or miss the Flagellant takes damage equal to their bonus Rage Damage + their Constitution Modifier. The damage type is the same as their weapon and it can be mitigated by resistance. The Flagellant may choose to make a number of bonus attacks equal to the number of attacks they made during their Attack action. Each one of these bonus attacks will deal damage to the Flagellant.
Burden in Pain
At sixth level, when a creature you see, makes a melee attack against a Large sized or smaller target within 5 feet of you, as a reaction a Flagellant can interpose themselves between the two. The attack is treated as having disadvantage and if it misses the initial target, it strikes the Flagellant who suffers damage of the attack. If the attack hits the initial target, then the initial target takes only half damage (rounded down) while the Flagellant takes half damage (rounded up). Creatures over Large size can't be protected.
When a creature you see, makes a ranged attack against a Large sized or smaller target within 5 feet of you, as a reaction a Flagellant can interpose themselves between the two. The initial target is treated as having 3/4 cover and if it misses the initial target, it strikes the Flagellant who suffers damage of the attack. If the attack hits the initial target, then the initial target takes only half damage (rounded down) while the Flagellant takes half damage (rounded up). Creatures over Large size can't be protected.
So Much Left to Give
At tenth level, once per long rest a Flagellant can utilize their resource pool from Comfort in Pain to restore health to an ally. As an Action the Flagellant must target a creature within 5 feet of themselves that is alive. The target receives healing equal to half of the pool value (rounded down). The pool value is equal to the Flagellants maximum health minus their current health. When this action is used, as a Bonus Action the Flagellant may also regain health equal to half of the pool value (rounded down).
So Much More to Take
At fourteenth level, once per long rest a Flagellant can utilize their resource pool from Comfort in Pain to deal massive damage. When the Flagellant performs an Attack and hits a target with the very first melee or thrown weapon attack on their turn, they can add bonus damage equal to half of their resource pool value. The pool value is equal to the Flagellants maximum health minus their current health. The target can make a Constitution save to take half damage. The DC is for this save is 10 + the Flagellants Constitution Modifier. When this action is used, as a Bonus Action the Flagellant may also regain health equal to half of the pool value (rounded down).
Comfort in Pain
At third level a Flagellant gains a resource pool which is equal to their maximum health minus their current health. Once per long rest, as an Action a Flagellant can utilize this pool and regain health for half of the pool value (rounded down) . When this action is used, as a Bonus Action the Flagellant may also regain health equal to their Constitution Modifier.