The Clone Spell

 Clone is an 8th level spell available to Wizards. And it is...

Let me put it this way: why would you want to become a lich when this spell exists?

(In fact, there are plenty of reasons - one being that even if your foes find your phylactery, they don't necessarily know how to destroy it, and there are some inherent advantages to being an undead being, including some damage resistances and natural armor.)

But let's look at this:

You need a fair amount of gold to cast this spell, between reagents and a vessel to keep the product. There is, also, a somewhat uncomfortable surgical procedure that you need to undergo to provide the main material component - a cubic inch of your own (or the target's) flesh.

Now, a cubic inch is not like Shylock's pound of flesh - you could cut it out of a chunk of muscle that would be likely to heal - probably with a scar, though of course if you're a high-level wizard you likely have a healer friend who could fix that right up.

Using this biological sample and the other components, you start a clone cooking - a physical body that is inert and soul-less. The body grows to maturity after 120 days, but one of the really amazing things about this is that you can set its apparent age to be younger than the donor. Once matured, it remains intact and inert indefinitely.

If the donor dies after you've cast this spell, their soul transfers into the cloned body, and they awaken, alive again. With the soul as well as all the memories and abilities of the original donor, it's genuinely that person. The old body is now inert and incapable of being revived, because that person, well, is already alive.

This spell is immortality. There is no rider that says "this can't extend the person's lifespan beyond its natural length." A 95-year-old archmage could make a clone body with the physique of an 18-year-old and, if they die - and they might just die of natural causes - they just wake up, literally rejuvenated.

This is pretty crazy.

This is also one of those spells that I think is designed more for an NPC (often a villain) than a player. There are a few reasons why you might not want to cast this:

The first is that you need a secure place to store your clone. If your clone-vault is discovered, you might not realize you're far more vulnerable than you thought you were - your foes might destroy the clone body and thus rob you of your resurrection, or they might keep that body securely restrained and kill your original body so that you are now fully in their custody.

The second is that this pulls you away from what you might have been doing. If you're fighting the big bad of your campaign - say, an Aspect of Tiamat, and that breath weapon fries your squishy wizard, normally the Cleric might want to run over to your corpse and pop you up with a Revivify. But they can't, because you've just been taken out of the fight (well, barring something like the Teleport spell).

You also leave all your gear behind, including costly material components for your spells.

All that being said, though, this is the perfect "retirement" spell for a wizard to learn. If they aren't going adventuring anymore, a wizard could use this to reset their body to a state of youth - technically, they could become a child again.

Just as a lich should probably leave a spellbook and a spell focus next to their phylactery, a wizard who uses clone should probably do the same for their clone. But I will note that an Order of Scribes wizard could actually just leave a blank spellbook there and draw their awakened spellbook's consciousness into the new book along with all their spells.

Notably, this can be cast on creatures other than yourself. As long as the creature is willing, you can make a clone of them. So an entire adventuring party could be preserved in clone form.

This is an 8th level spell, so players will have to be very powerful to get to a point where they can use this spell, and its odd nature does make it seem like it's intended more for NPCs.

The obvious reason to use this is so that you can have a powerful wizard come back even if the party defeats them. That's a major part of the appeal of the Lich as a monster, but this gives you something of a Lich-light.

But I actually think the similarities could make for a really interesting epic conflict in the background. While this is a necromancy spell, it does not require the sacrifice of anyone's soul and it does not render the caster some kind of horrible undead abomination.

So, I think this could be a very interesting spell to give to the campaign's "Big Good."

Imagine the party getting swept up in an age-old conflict between a semi-immortal archmage and a dastardly lich. The two have been playing a high-stakes game of chess for the fate of the world, with the archmage trying to preserve freedom and life while the lich wishes to enslave the world and rule with an undead army.

Now, the party has the archmage as some kind of group patron - an authority figure who guides them on their quests. But the very vulnerabilities of the Clone spell make it potentially great as a plot device. At some point in your campaign, after the party has gotten used to the archmage's help, the party is asked to safeguard the wizard's tower while he goes on some important mission. While that's happening, the lich sends a strike team into the secret room where the archmage's clone is hidden. The attack on the party has all been a distraction while the real goal was destroying the clone.

With the clone destroyed, the lich brings his full might to bear on our archmage patron, and kills them. Without the clone, the archmage is gone, and the lich is poised to take over the world after thousands of years - unless the party can step in and stand in his way.