Gaining the lich template will make the character more powerful than a normal creature of his level correct?
Read my last two posts if you disagree.
Now, if the creature is more powerful than other creatures of his level, we run into a few minor problems. First off, CR's are based off the assumption that characters of X level have Y cababilities and Z weaknesses.
So CR's are off. This also means that XP awards are off.
So how do we fix this? Well, we can figure out about what level his character is equivalent to (i.e. ECL) and then the numbers work out, but then there's the matter of current and new XP amounts. That can be taken care of in any number of ways.
But saying the character is the same level or 2 levels higher will not address the problem.
This will lead to both the DM and the players besides the player of the lich having less fun.
Put it this way, would you allow one of your 3 10th level dwarf PC's to become immune to fire and acid damage, up his hit points, gain the ability to respawn upon death, make him immune to three schools of magic including most will saves and most fort saves, and give him the ability to cast a DC 24 hold monster spell at will that has a permanent duration, while also giving him armor of heavy fortification to make him immune to all sneak attack and critical hits, giving him a couple levels of paladin to make him immune to all disease, give him a couple levels of druid to make him immune to all poison, give him a couple levels of barbarian to give him DR, and up his primary stats while gaining +8 to 6 skills like search, spot, listen, sense motive, etc.?
Would you let a PC start with those abilities? Why not?
It's unbalancing and not fair to the other PC's.
The objective of ECL is to take a PC and make him equivalent to other PC's in his desirability. So that no one PC is more important than the others. Mess with that balance at your peril.
Adding XP pitfalls or taking or adding levels is a distasteful thing to do. But if you allow those traits to suddenly spring into being upon the completion of a ritual, they've effectively gained 6 levels in a pretty kick ass prestige class. Hell, sign me up for any class that gives that many immunities to melee, ranged, monster, and spell combat. I'd take it just for the immunity to energy drain, ability damage, and sneak attack. Even if that was the 6th level ability.
This is too many POWERFUL powers to give to a PC without affecting his effective level. A lich wizard 12 is not the same as a elf wizard 12. Not by a long shot. They shouldn't be awarded XP as such, challenged as such, or affect the party level as such.
But if that's what you want to do.. Me personally, if I were the DM and my PC wanted to step out there and do something extraordinary and noteworthy such as that, I would sit down with him and discuss a whole lot of things.
I'd pare the class down to the abilities he wanted and shave a little ECL off, then I'd compare him to the rest of the party and assign an effective ECL. Then as others have suggested I'd make that ECL into a prestige class goal. No one said the ritual didn't take 3 years or 3 adventures to complete. And as he gains levels in his prestige class, his ritual nears it's completion and his transformation gives him new powers as he goes along.
Much preferable to , POOF I'm virtually immortal!
That's my take on things.