I'm no MODmaker. But wouldn't adding a lich in the party suck out the fun of the game? I mean heres my example of a Lich battle:
Dierix casts:Spell:Time stop
Dierix casts:Spell:Symbol Fear
" '" " " " " " " Stun
" " " " " " " " Death
Dieric cast:Spell:Wail of the banshee
Mazzy

eath
Aerie

eath
Viconia: Save vs Spell
Viconia: Death
Minsc:Save vs spell
Minsc save vs spell
Minsc:Stunned
Edwin:Stunned
Edwin: Death
Ballchiled: Save vs spell
Ballchiled:save vs spell
Ballchiled : Save vs spell
Dierix casts:Spell:Finger of Death: Minsc
Minsc: Death
Ballchiled casts:Khelbens wharding whip

ierix
Dierix:Spell protections removed
Dierix casts:Spell: Flesh to stone: Ball chiled
Ballchiled

etrified
GAME OVER
As I have just explained lichs, if used properly, they can defeat ANYONE!
Becaus they are supposed to have unlitimed bank of spells.