Martyr thorns are long (1/2  to 1 inch) thorns harvested from a few rare species of carnivorous vines and imbued through necromantic rituals with magical power. 

The thorns are useless themselves. But when affixed to the hilt or grip of a melee weapon (being set into the metal, woven into a leather wrap, etc.), they provide combat benefits to the user--with a small price.

Upon drawing or using a thorn weapon, you must impale your hand upon the thorns. The thorns are typically placed so as to make wielding a thorn weapon impossible without impaling your hand in this way. Thus drawing or using the weapon initially inflicts 1 point of damage to the wielder. Thereafter, once per round you may elect to inflict another point of damage to yourself by instructing the thorns to painfully drain a small amount of your blood. If you suffer this point of damage, you may avail yourself of the benefits  below (depending on thorn weapon category). If you avoid taking 1 point of real damage for any reason or by any method (e.g. the damage coming off magically granted temporary hit points, or magically transferred to another person by some means), the benefits are unavailable. You must suffer the damage to use the weapon to its fullest.

Activating a thorn weapon in this way does not require an action, merely a mental command. 

Lesser Martyr Thorn Weapon

These are weapons created through necromantic magic, requiring small amounts of blood to be sacrificed in the creation process. The blood 'donor' easily survives. When you have activated a lesser thorn weapon, you receive a +1 bonus to your attack roll on the first attack after activation (i.e. after losing the 1 hit point). Furthermore, if the attack is successful, the victim must make a charisma save (target is 12+wielder's charisma or wisdom bonus) or be wracked with agonizing pain. This pain results in all activities performed in the victim's next turn being made at disadvantage. On a successful save, the victim still experiences intense pain, but is able to fight through for no game effect.  

Greater Martyr Thorn Weapon

These weapons are created through the same rituals as lesser thorn weapons, but require the sacrifice of a life (by means of draining all the blood from a living sentient being). Activating a greater thorn weapon provides you with the same bonuses to your first attack as a lesser thorn weapon. In addition, greater thorn weapons also allow you to inflict extra damage. By sacrificing an additional 1 hit point of blood to the weapon when activating, you inflict an additional 1d6 of psychic damage with a successful hit. You may elect to sacrifice yet another point of blood (3 total) for an additional 1d6 of damage (+2d6 total), but will then suffer disadvantage yourself on any actions during your next turn due to the pain.

Notes: Upon first wielding a specific martyr thorn weapon, you must use it in combat for 2 rounds before being able to make use of its benefits. You must also be a, sentient being with blood (or blood equivalent) and the ability to feel pain.