How to trick my players into helping someone become immortal without them becoming immortal themselves?

[L. A. DuBois]

So my players have goaded me into making Sun Wukong as a recurring antagonist for them. However, it seems to me that it would be far more enjoyable - for everyone involved - if he isn't already SUN WUKONG when they first meet him. And if he's not already SUN WUKONG, then why not have the players be unwitting instruments in turning him into SUN WUKONG?

And so, has anyone got any ideas for how I can set up a scenario where my party unwittingly helps a mischievous monkey unwittingly become immortal?

[Veltharis]

If you want to keep them in the dark about the nature of their quest, I'd say make the immortality-inducing McGuffin a single-use consumable - sacred Golden Apple stolen from the gods, only known vial of the Elixir of Life, etc. - that they can find, but not replicate. Ensuring it winds up in Sun Wukong's hands rather than your PCs' will require GM finessery, but that would probably be your best bet.

If you DON'T want to keep them in the dark, I'd say go full Planescape: Torment and make it abundantly clear to them that immortality is not worth the price of entry.

[Pizza Lord]

Obviously it's your story, and I know nothing of Sun Wukong other than what I just looked up on the wiki. I guess he was originally just a stone statue, then he was turned to a monkey, then became the Monkey King, then learned a lot of stuff from the god of magic, Qi Zhong. Then he snuck into Pharasma's Boneyard and erased his name from her records and thus became immortal.

So theoretically, if you somehow tricked the PCs into erasing his name, that would work. Possibly they plan to do theirs as well, but find his first and erasing it triggers 'alarms' or guards or attracts attention and they don't have a chance to find theirs. Or they think the name belongs to someone else or has a different meaning.

If you want to go with your own immortality ritual, possibly there's an altar, an artifact dagger, a special time of the year, etc. If the characters all stand around an altar and sacrifice a monkey with the artifact dagger they gain great power. Except... the Monkey King tricked them, turns out the dagger drains power from those around the altar and imbues it into the one whose heart it's stabbed into. So they get weaker or cursed with some effect and he gains immortality, thus also explaining their enmity towards Sun Wukong... and his towards them, after all... they were willing to kill him for power.

[Chakat Firepaw]

Another possibility would be to make it a multi-stage process and he's already on step 3 or 4. Sure the PCs could duplicate what he does, but first they would have to figure out what the preparatory steps even are so that they can do the things that make the rest of it work, (or even be survivable).

Going with the unwitting instrument thing also brings to mind the idea that some step requires the person doing it not know. Sun Wukong is using the PCs as cat's paws because he literally can't do it himself and once the PCs know what is going on, neither can they.

[Guru-Meditation]

What kind of "Immortal"?

* Ageless, eternal youth?

* Invulnerability to (almost) all weapons?

* Reformation after destruction?

A combination of the above?