You can only gain this item after undergoing the ritual to become a lich. After undergoing a 48 hours long ritual undertaken as outlined in the Book of Vile Darkness, in order to become a lich the ritual requires.

• The drained blood of five innocents, killed by your hand
• The bones of your ancestor, to bind their soul (which is irreversibly consumed) as energy for the ritual
• The beating heart of the caster, removed in the ritual
• The horn of a metallic dragon
• A vial of ghost ectoplasm
• Four vampire fangs, pried from the mouths of conscious vampires
• One vial of hydra venom
• A vial of gorgon blood, specifically from the left side
• The item that becomes this phylactery (an object to hold one’s soul)

Undead. You become undead in addition to your other creature types.

Turn Resistance. You have advantage on saving throws against any effect that turns undead.

Lich's Armor. If your AC would be lower than 17, it becomes 17.

Lich's Resistances. You have Resistance to Cold, Lightning and Necrotic damage as a lich.

Damage Immunities. You are immune to Poison, Bludgeoning, Piercing, and Slashing from Nonmagical Attacks.

Condition Immunities. You are immune to the Charmed, Exhaustion, Frightened, Paralyzed, and Poisoned conditions.

Lich Sight. You gain Truseight to 120 ft.,

Rejuvenation. Whilst this phylactery survives, you gain a new body in 1d10 days, regaining all your hit points and becoming active again. The new body appears within 5 feet of the phylactery.

Paralyzing Touch. Your unarmed attacks deal 10 (3d6) cold damage. On a hit, the target must succeed on a DC 18 Constitution saving throw or be paralyzed for 1 minute. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Notes: Immunity: Poison, Immunity: Bludgeoning, Piercing, and Slashing from Nonmagical Weapons, Resistance: Cold, Resistance: Lightning, Resistance: Necrotic, Immunity: Charmed, Immunity: Exhaustion, Immunity: Frightened, Immunity: Paralyzed, Immunity: Poisoned, by a creature capable of casting 9th level spells