excusev me if this is more of an in game topic but it feels likt there could be a littlev more info ooc to help direct
a necromancer can become a mummy
is this declared what he ants in his role
do you pray about it like empoerment orjust let someone come to you f they think you are worthy of it
is it common you get your quest

a lich still a thing ?
are there any others?
wight?

what are the difference in their powers?

http://www.carrionfields.net/helpsearch.php?keywords=becoming

This is about all the information you'll get for most purposes.

Otherwise if you ask around in game as a necro, you'll definitely get some hints/tips about what you might need to do.

Aside from that all I'll say is:
Lich - hardest, prays required in a certain fashion.
Mummy - Still difficult. No praying required, just a quest.
Wight - easiest.

You could know ahead of time what you're aiming to do, or figure it out later. Depending on your level of masochism you could pick accordingly, but shouldn't expect success :)

-Ish

I'll also add that the first two have a significant chance of failure, which would result in the permanent death of your character.