You can choose between +2 to dexterity OR +2 to constitution, the other one is increased by 1. You also have to choose between a -1 to intelligence OR a -1 to wisdom.

you are essentially immortal, you cannot die of old age. often a true deathless only is created after about 3 or 4 resurrections.

You know undead and your undead origins languages, undead is a speech witch cannot be learned by the living. it is, for example, the groaning you hear of zombies, these groans often are just cries for help or screams of agony. it cannot be written down or cannot be understood using magic.

your speed is your undead origins speed -5

A deathless comes back as a deathless after being revived, raised or after waiting 100 years (the last one being rare). if the corps of a deathless is influenced by reincarnate, only the undead origin is changed.

You have resistance to necrotic damage, resistant to poison damage. But are greatly weakened to fire damage (you take 3 times fire damage)

you know the Spare the dying cantrip. you can banish people from death as you were, all those lives ago

When you are LV3 you learn the spell Gentle Repose, you can use this spell once per long rest. so they may not be cursed as you were.

you now have to choose a race which is your undead origins. this can NOT be an undead, construct or any other (technically) nonliving creature.

From your undead origin you gain; your size, your speed(ceep in mind, you have to subtract 5), your weight and your languages(you also know undead)... you do NOT gain traits, but you DO gain attacks like a dragonborns breath attack...

your appearance is also dictated by your undead origin... the only difference is that your eyes now have a green glow (but still retain there original colour), your fingertips and eyes now sprout black "Roods", this is only a change of skin colour however.