5e Homebrew Spell: Lichdom

The following spell allows evil aligned player characters to participate in the creation of liches, if they so wish. It is not recommended that characters be allowed to learn this spell through leveling. Instead, it should only be accessible in the form of spellbooks and scrolls owned by liches who were once wizards. A good wizard might find a lich's spellbook, discover this spell is contained, and have it destroyed, while an evil wizard might copy the spell into their spellbook. Neutral wizards might try to preserve, protect, and hide this information. Truly evil wizards might actually cast the spell.

If you do allow players to access and cast this spell, use it to create new villains for future adventures and campaigns. It is, essentially, a fairly expensive way of retiring a PC to create a villainous NPC for the DM to use in the future. It works best in sandbox campaigns where the players are the primary source of action and conflict in an emergent narrative.

Lichdom

9th Lvel Necromancy (Ritual)
Casting Time: 1 Year
Range: Self
Components: V, S, M (A small box or container worth 100,000gp, silver worth 1,000gp, and poison mixed with the blood of a sacrificed good-aligned humanoid which is consumed by the spell.)
Duration: 1 Year
The character who casts this spell contacts a powerful entity, likely orcus or some other fiend or god of death or undeath. The caster makes a pact with the entity, at the signing of which the entity delivers the secrets of immortality through undeath. (Work with the player to write up a contract-like pact for their lich to be bound by.) At the end of the casting time, the caster sacrifices a good-aligned humanoid to their phylactery, a box inscribed with silver runes, mix their sacrificial blood with poison, and drink the concoction, dieing immediately. The player then makes a DC15 arcana check. If they succeed, the dead character is then replaced by a lich NPC, as it appears in the Monster Manual. If they fail the check, the dead character is instead replaced by a Boneclaw NPC as they appear in Mordenkainan's Tome of Foes.
Lichdom can be extended for 1 year beyond the end of its current duration by casting the Minimus Containment form of the Imprisonment spell, using the phylactery in the place of a crystal, and keeping the target of that spell imprisoned for at least 24 hours. Casting dispell at 9th level on the phylactery within 24 hours of a creature being trapped in it will release that creature; otherwise, the creature's soul is destroyed by the phylactery, preventing it from being restored to life. The phylactery can contain only 1 creature at a time under the imprisonment spell.
If the phylactery is ever on a different plane than the lich, lichdom immediately ends.
When lichdom ends, the lich decomposes and is replaced with a demilich as it appears in the monster manual.
If the creature casting this spell is a dragon, it instead becomes a dracolich. If lichdom ends for a dracolich, the creature simply dies rather than becoming a demilich.

Spell Lists:
Sorcerer
Warlock
Wizard