Ki is the energy of life flowing through the body and soul. The crimson soul corrupts this. They fill their souls with negative energy, the bane of all life. With it, they can steal life energies from others in order to heal themselves and use the bodies of the dead as puppets. 

However, the negative energies have left them with altered bodies. They are stronger fighters for it, but their path has left them susceptible to light and the power of radiant energies.

Blackened Bodies

At 3rd level, when you choose this tradition, your use of the negative energies has left your body changed and altered. You are now more dead than alive and as such you appear as an undead when sensed by abilities and spells such as the grave cleric's eyes of the grave. You are also vulnerable to radiant damage, and you only regain half the hit points you would when you are healed by spells and abilities that come from positive energy such as cleric or paladin's spell.

However, your undead essence grants you the following benefits. You have resistance to cold and necrotic damage, and whenever you use your Flurry of Blows ability against a creature, you gain hit points equal to the damage dealt unless the damage is dealt to a construct. 

Sparring Puppet

At 6th level, you begin to dabble in the necromantic elements of negative ki. If you drop a large or smaller creature to 0 hit points, you may spend 2 ki point to resurrect them on the spot. They are now undead, and will maintain their initiative order as normal, but you can command them on their turn.

When a creature is resurrected this way, roll three of your martial arts dice for a small creature, five for a medium, and eight for large. Your puppet gains that many temporary hit points. Your puppet remains reanimated for one minute. You may spend 1 Id point to maintain them for each additional minute afterward. You may command them to attack, move and use abilities that you've seen them use, but they may not cast spells.

This feature does not work on constructs or creatures that don't have a body to resurrect such as a fire elemental.

Death Endurance

At 11th  level, you gain a certain unkillableness granted by your undead nature . Whenever you take damage, you may spend a ki point to take half the damage. 

Life's Bane Touch

At 17th level, your body has become conduit for negative energy, and a mere touch of your hand can kill. Anytime you make a successful unarmed attack, you may spend 3 ki points to deal 4d10 necrotic damage to the target you hit. If the target creature is below 50 hit points when you use this feature, you automatically reduce it to zero hit points. You may only do this once on each of your turns.