Lich from the imprisonment machine...

[Assassin]

Who says the Lich NPC has to be overpowered? Sure, she'd be immune to Normal Weapons and spells from Levels 1-5, but she'd have the accompanying weaknesses like intolerance of sunlight, inability to be Raised or Resurrected, and susceptibility to Turn Undead and anti-Undead weapons & spells.

Intolerance to sunlight is really the only thing that I would worry about. Inability to be Raised or Resurrected isn't that much of a problem, as by the time I reach the Underdark, as long as I use semi-decent tactics, no one in my party should die.

The advantages I considered giving her: Alchra Diagott will try to cast Imprisonment on you, and I don't think it's from a scroll. Therefore, we can presume she knows Level 9 spells, including Time Stop, as just about every Lich in the game knows Time Stop. For a party that rushed to Spellhold, this would be a huge bonus. Liches sacrifice a lot to gain their immortality--their Unlives are now dedicated wholly to the study of arcane magics. Therefore, they should get a bonus spell per level, without having to choose an Opposition School. (We know Liches aren't Necromancers, since they love to cast Illusion spells.)

Erm... So, you're proposing to allow a level 18 Mage to join you in the Underdark?

The disadvantages I considered giving her: Cannot be Cured, Healed, Raised from the dead, or Resurrected. Her only means of regaining lost HP is via a Regeneration item or her own natural Regen. If she gets hurt to less than 10% of her HP, she will discorporate and waft away to her phylactery to Rest and Heal. She won't come back until you Rest, and then you'll have to convince her to join the party all over again. Cannot use weapons. When's the last time you saw a Lich hit you with a Staff or a Throwing Dagger? That damn movement rate--she needs the Boots of Speed just to keep up. Try running away from the nasty Dragon with a Lich in the party. -4 Reputation penalty when she joins up. Will refuse to join if your party contains any Neutral Clerics, or ANYONE of Good alignment. You can't Romance her. (Ewwwwwww!)

Tell me again, what is the point of not allowing her to be Raised or Resurrected if she discorporates? And, IIRC, Liches every hit requires a save versus spells or be paralysed, or something.