Classes that suit a demilich?

[Sordahon]

I want to make a demilich with high epic levels(~100 but without epic spells and broken normal spells) and thought what classes it could have in flavour, I thought about wizard, dread necromancer, true necromancer, soul eater but some of these have abilities which cover each other, my idea is about a demilich casting as wizard, good undead boosting feats/powers, and something which could make demilich trap the soul better, as it is now, it's just killing someone and devouring his soul with no benefit really, I thought about demilich being able to hold soul imprisoned instad of devouring it(which has no benefits) and supplying it with soul eater class that can do energy drain as touch attack and gives benefits(saves, stackable ability bonus etc. for 24h) if you eat the soul(drain it to death) then applying it to devouring soul, so I could make the demilich trap soul, have a few souls as victims waiting to be devoured and buffing the demilich when needed. What do you think?

[Inevitability]

You're talking about a level 100 character, so I suggest getting 20 caster levels in a couple of caster classes (maybe facilitated with dual-progression prcs), throw on your epic level PrC of choice, and topping it all off with as many epic spell bonuses as possible.

What is this character supposed to do? Is he supposed to be just a fun theoretical exercise, or (god forbid) an actual challenge for high-level PCs?

[Sordahon]

You're talking about a level 100 character, so I suggest getting 20 caster levels in a couple of caster classes (maybe facilitated with dual-progression prcs), throw on your epic level PrC of choice, and topping it all off with as many epic spell bonuses as possible.

What is this character supposed to do? Is he supposed to be just a fun theoretical exercise, or (god forbid) an actual challenge for high-level PCs?

An exercise, so I am not min maxing or anything, just flavor prc/classes/powers that I could take to fit him as some kind Harvester of Souls where he has undead army, and gets buffs from eating souls. Also can I have permamently controlled intelligent undead? I don't know if rebuke does it, or like create greater undead or animate dead + awaken undead. Something like Larloch with 60 liches servants.

[CIDE]

Something at level 100 could have something really nasty built up via Leadership or one of its variations.

[ShurikVch]

I want to make a demilich with high epic levels(~100 but without epic spells and broken normal spells) and thought what classes it could have in flavour, I thought about wizard, dread necromancer, true necromancer, soul eater but some of these have abilities which cover each other, my idea is about a demilich casting as wizard, good undead boosting feats/powers, and something which could make demilich trap the soul better, as it is now, it's just killing someone and devouring his soul with no benefit really, I thought about demilich being able to hold soul imprisoned instad of devouring it(which has no benefits) and supplying it with soul eater class that can do energy drain as touch attack and gives benefits(saves, stackable ability bonus etc. for 24h) if you eat the soul(drain it to death) then applying it to devouring soul, so I could make the demilich trap soul, have a few souls as victims waiting to be devoured and buffing the demilich when needed. What do you think?Dragon Magazine #322 have Soul Reaper - variant Necromancer class; it sounds like the exact thing you need:
Spiritual Vassal – The Soul Reaper can perform a ritual (that includes sacrificing a humanoid) to summon a Spirit to use as his/her familiar. The Spiritual Vassal looks like a ball of light (similar to a Will-O’-Wisp), but can grant its master temporary skills, languages, & feats gleaned from the afterlife. Follow the Familiar rules on (DR312 p36).
Bind Spirit – The Soul Reaper can trap the Spirit of a creature in a 500gp gem. The Soul Reaper must Ready the action for the moment the target dies (i.e., –10 hp, etc.) & it receives a Will save to avoid the effect (DC = 15 + Intelligence modifier). If trapped, the target cannot be restored to life, etc., until its Spirit is free again (either by 1 day per Class level passing or by breaking the gem). The Soul Reaper may do one of the following once per day: get the answer to a “yes” / “no” question, speak one of the spirit’s languages for 1 hour, or use the ranks of one of the spirit’s skills for 1 minute.
Spawn Undead Servitor – The Soul Reaper imprints a Skeleton or Zombie with a Spirit trapped by the Bind Spirit ability. The Undead gain the Spirit’s Intelligence, Wisdom, Charisma, Base Attack Bonus, Skill Ranks, Weapon & Armor Proficiencies, & Feats. The new Undead Servitor has the Soul Reaper’s alignment, but must be controlled normally. The Spirit used to make the imprint is freed once the process is finished.

[DarkSoul]

The kinds of things you mention wanting to do can most easily be achieved with Epic Spellcasting. Just don't abuse the rules like is so commonly done. Make spells with actual DC's, research times and costs, etc. You're going to have a ridiculous amount of gold to spend anyway.

Either go Charisma casting and find a way to give this demilich Unholy Toughness, or use an epic spell to grant it an Intelligence version. I wonder if there's a way for it to use the Corpsecrafter feats on itself...

If you go the undead leadership route and can get enough epic-level followers, you could always make this guy the skull and have 5+ other demilich followers/thralls as arms, legs, and torso in order to masquerade as the most overpriced skeleton ever.