You unlock the pinnacle of necromantic prowess, through which you conquer death itself: The Rite of Lichdom.
Becoming a Lich
The Rite of Lichdom is a ritual that you can perform over the course of 30 days in an isolated location. Once completed, your soul is bound to a spirit jar and you become a lich, an Undead of incredible power. You gain the following benefits, as well as those granted by your Necromancer subclass.
Creature Type
You are Undead. You are immune to any effect that turns Undead.
Spirit Jar
A spirit jar is an object that houses a lich's soul, safeguarding its immortality. If you drop to 0 Hit Points, your body crumbles to dust and you reform after 1d4 + 1 days, reviving with all of your Hit Points. Your new body coalesces in an unoccupied space within 5 feet of your spirit jar. The new body is identical in every way to the one that was destroyed but has none of the original's equipment.
Truesight
You have Truesight with a range of 120 feet.
Undead Immunities
You have Immunity to Necrotic and Poison damage and to the Exhaustion and Poisoned conditions.