My RAW Necromancer (pure Wizard) wants to become a Lich.
What's the surest path to becoming one?
He is OK to change alignment and spend lots of coins, but not to worship any god or sell his soul with an infernal contract.
By RAW, the surest way to become a Lich is to use the occult ritual : Eternal Apotheosis
The main problem is to find a way to learn this ritual.
Depending on your GMs approval, a DC 24 Intelligence check might be enough to learn it from scratch.
Learning a ritual from hidden clues or from scratch takes a week or a month per ritual level (GM’s discretion). At the end of this period of study and contemplation, the person attempting to learn the ritual must succeed at an Intelligence check (DC = 15 + the ritual level if learning from clues or a coerced teacher, or DC = 10 + the ritual level if learning from an instructor eager to teach).
If you GM approves, this is the easiest way.
If you GM does not approves, there's a longer but surer path.
Who could teach it without taking your soul or devotion in payment ?
Your surest bet is the Greater True Name Feat (taken at lvl 15)
With this feat, you can learn the true name of a Lilitu Which has just the right number of HD to be called in this manner.
As per the True Name description :
It must obey you to the best of its ability, without pay or bargaining for its services, for its fear that you might release its true name to the wider world is enough to bring even the most recalcitrant of outsiders to bear.
Which means the Lilitu will do everything in her power to help you for free.
Now how can you be sure that the Lilitu knows this Ritual ?
Well. Orcus, the arch demon of undeath and patron of Liches happens to have lots of Lilitu at his service
In his Abyssal realm of Uligor, Orcus is served by strange demon-undead hybrids, powerful liches, and lilitu demons.
Also, Lilitu are demons of sin, they will try to push mortals to commit sins by granting them Wishes. And becoming a Lich is typically the kind of "sin" Orcus encourages.
So, use the feat to find the True Name of a Lilitu that worships Orcus and ask her to use her "Wish" ability to explain you the Eternal Apotheosis ritual and threaten to spread her True Name if she doesn't. (or tries to trick you in any way.)
Once the Occult Ritual is learnt, all you have to do is perform the ritual and make your Phylactery.
Beware that the ritual is inherently a heinously evil act and involves a number of DC 34 skill checks. And in case of failure, your character will turn into a forsaken Lich and die in matter of months.
This is the safest path I could find to Lichdom, which doesn't mean that it is safe at all. The Lilitu might try to get revenge on you and purposely give you a false ritual destined to fail or come during the ritual to prevent you from succeeding in it's accomplishment, all of this is at your DMs discretion of course.