This is my attempt at providing lichdom to players via epic boons. I don't intend to comment on what kind of quests a PC would need to undertake to earn these boons; that's setting-dependent anyway. The idea is that you need to collect several boons before you can call yourself a lich, which represents the fact that the 'average' lich is ancient in years and vastly more experienced than even a 20th level PC should be. Without further ado, here are the boons. Let me know what you think!
Boon of Epic Necromancy
Prerequisites: able to cast 9th level spells, know at least one necromancy spell
You gain one 10th level spell slot that can only be used to cast necromancy spells of 9th level or lower. When cast at 10th level, the spell create undead allows you to create or reassert control over 8 ghouls, 4 ghasts, wights or mummies, or 2 revenants or wraiths.
Boon of Lichdom
Prerequisites: boon of epic necromancy, boon of unspeakable knowledge, access to the spell Imprisonment
You perform a terrible ritual to become an immortal, undead abomination. You can no longer die of old age, though your flesh continues to age unless you take steps to preserve it. Your creature type changes to undead, you can no longer benefit from healing magic and you don't require air, food, drink or sleep. You are immune to the charmed, exhausted, paralyzed and poisoned conditions. Finally, you gain a phylactery (see below), which enables you to regenerate your body even if it is destroyed.
Boon of Mastery Over Death
Prerequisites: boon of lichdom
Years of undeath and layer upon layer of long-lasting abjurations have hardened your physical form. Your armour class is equal to 14 + your dexterity modifier, you have advantage on saving throws against any effects that turn undead and you are immune to poison damage and bludgeoning, piercing and slashing damage from nonmagical weapons.
Boon of Unspeakable Knowledge
Prerequisite: non-good alignment, able to cast 9th level spells
Having unraveled the secrets of lichdom, you gain the ability to disrupt the life energy of other creatures with your touch. Your unarmed strikes now deal 3d6 cold damage and you may use your spellcasting ability instead of Strength for the attack rolls. Furthermore, you may choose to either force a target you hit with an unarmed strike to make a Constitution saving throw against your spell save DC or be paralysed for one minute (the target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success), or drain the target's life, gaining temporary hit points equal to the damage dealt.
New Magic Item
Wondrous item, legendary (requires attunement by a lich)
A phylactery is a small object, usually a locket or box, that houses a lich's soul. If a lich is reduced to 0 hit points, their body crumbles to dust but their will and mind escape to the phylactery. After 1d4+1 days, a new body for the lich coalesces as near to the phylactery as possible. The new body is identical in every way to the one that was destroyed.
However, the lich must feed souls to its phylactery to sustain the magic preserving its body and consciousness. It does this using the imprisonment spell to magically trap the target's body and soul inside its phylactery. The phylactery must be on the same plane as the lich for the spell to work. A lich's phylactery can hold only one creature at a time, and a dispel magic spell cast as a 9th-level spell upon the phylactery releases any creature imprisoned within it. A creature imprisoned in the phylactery for 24 hours is consumed and destroyed utterly, whereupon nothing short of divine intervention can restore it to life.
A lich that fails or forgets to maintain its body with sacrificed souls (at least one soul every ten days) begins to physically fall apart, taking 10d10 radiant damage at the end of the tenth day and being unable to recover hit points until a soul is fed to the phylactery, and might eventually become a demilich if it does not feed its phylactery for a year and a day.
Every phylactery has a unique weakness that allows it to be destroyed. Determine this weakness by rolling on the table below or choosing your own.
|d10||The phylactery can be destroyed by|
|1||Casting it into an active volcano|
|2||Submerging it in holy water for 24 hours|
|3||Smashing it with a weapon of legendary rarity|
|4||Opening it with a specific key|
|5||Taking it to Mount Celestia|
|6||Solving a puzzle-lock on the item|
|7||Performing a 1-hour ritual known only to the creator|
|8||Saying a secret passphrase|
|9||Playing it a certain song|
|10||Bathing it in the blood of a demon|