I'm trying to create a BBEG that can't die naturally, or at least lives for thousands of years. The PCs need to be able to kill them in the end though. I know I can use a Lich or Vampire but is there any other way to accomplish this?

Have them be a spellcaster with access to high level spells

The easiest way to achieve effective immortality is to use spells. A wizard has access to several of them once they get higher level spell slots.

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  Sequester will allow the BBEG to put themselves into a state of suspended animation during which they will not age. This will allow them to span any amount of time without aging (though it will mean that they are out of action for that time).


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  Clone will provide the BBEG with fresh young bodies to use once their older body dies. With enough of these, they can last an infinite amount of time. See this Q&A for more discussion on this.


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  Wish obviously wish can do anything, including granting immortality. A wish for immortality is unlikely to be granted in a straightforward manner though and would be a good opportunity for the DM to implement some sort of curse or other downside on the BBEG as a result of their wish.

Have them be a warforged or a monster

Warforged, from Wayfinder's Guide to Eberron, are partially mechanical beings. For age, the rules say:

The maximum lifespan of the warforged remains a mystery; so far, warforged have shown no signs of deterioration due to age.

So a DM would be perfectly within reason to give one a lifespan of several thousand years or even more.

If you have the option of making BBEG a monster, devils are immortal (MM), hags are "virtually immortal", and an ancient dragon is listed as living for "over a thousand years".

Magic items

The Ring of Winter listed in Tomb of Annihilation will prevent aging.

You, as DM can also feel free to invent any kind of item or artifact that can prolong life as well. Just be sure you are fine with having it fall into the PC's hands or have some sort of plan to have them need to destroy it.

In fact...

Make something up

If none of the RAW options above satisfy you, make something up. You are the DM and you have the power to change your world to contain whatever effects you want. Perhaps, the villain found or made a powerful artifact. Or maybe they found a sacred site or ritual that allows them to be long-lived. The options are really endless and really, there shouldn't be much of a concern here because the long-lived villain is a common trope and is not likely to be a problem for players to accept even without explanation.

Or even better, you can mix and match from all these options to create the long-lived villain of your dreams.

Use a monster.

There are plenty of creatures available that live for very long times. I won't bother listing all of them, but to name a few:

Dragons live a notoriously long time and can transform into humanoid form.

Fiends (read: Devils) are generally ageless*1

I'm unaware of an age limit for Hags.

Alternatively, MacGuffin

If you're the DM and you want the BBEG to have lived for thousands of years, then he's lived for thousands of years. Literally just make up a reason (try to make it a good reason); demonic pact, other worldly blessing, unicorn blood, whatever.

If you're determined for a RAW humanoid... a handful of class mechanics

As another user mentioned, cloning can keep you young for relatively little cost.

Having a druid cast Reincarnate on you after you die. This is risky because you have to trust someone to actually cast the spell, which is harder for a BBEG to achieve.

Being an 18th level druid slows aging by 90% (1 year for every 10). Combine this with an Elf or Dwarf and you can easily live thousands of years.

Honorable mention: being the target of Imprisonment. You don't age while imprisoned.

*1 The Monster Manual calls them immortal