This is a long answer, as I'm showing my work. This answer explores Tom as monster.
Initial thoughts and research
4th edition completely divorces PC from NPC mechanics, which helps us quite a lot, as all we have to do is build an epic level evli dude, without worrying about PC interactions at all.
Supremely evil lich.
He will be a Solo, and to keep the fight interesting, we'll apply the Lawbreaker mechanics, so the PCs can't trivially stunlock him. For purposes of this discussion, we won't build the encounter as well, though with any solo, 3-4 different locations are quite important to keep things interesting.
Now, as monster classes go, he is easily a Controller, Leader.
With that said, let's first adapt the magical abilities from the wiki into interesting powers for 4e monsters:
- Magic Mastery: "Pushed the limits of magic farther than they have ever been pushed" and "wizard alive, and that even his most powerful protective spells and charms were unlikely to be effective if Voldemort returned to full power. "
Part of this will be the "lawbreaker" support, as he "warps the rules of reality" , but the "powerful protective charms" suggests some way of disabling player items. (extremely mean, but interesting as part of the climax of a campaign.)
- "In the films, Voldemort has shown the ability to contain spells and release them in a blast that resembles a shock wave."
This reads like a wonderful "Immediate interrupt" for damage redirection.
- "Voldemort was also a masterful practitioner of all three Unforgivable Curses, with a special affinity for the Killing Curse, having murdered enough people to create an entire army of Inferi. Ironically, this would be his final downfall."
This gives us the primary standard actions.
Infiri as shambling zombie corpses make great minions that auto-spawn around the dark lord.
"Avada Kedavra" as killing curse will be OMG (a technical term, actually) necrotic and psychic damage to a single target. Insta-kills are boring and, as HP in many ways represent "dodging" and expending energy, the massive damage makes sense. As an alternative, it could just be a "kill a bloodied target" which is... harsh.
"Crucio" as a pain curse is well-implemented. Ongoing save ends psychic damage with a nasty penalty to hit.
And "Imperio" is a dominate.
The wiki lists: "Lightning spells: Voldemort cast a powerful lightning spell up in the air when he tested the Elder Wand after stealing it from Dumbledore's tomb." which gives us a nice encounter power, perhaps part of the lawbreaker progression, or an aura.
"Master duellist" is... functionally a description of a solo
"Charms master" suggests invisibility and item teleportation.
"Fire spells" is... another elemental type. This theme suggests that one of these spells should be used to signal each stage of the fight.
"Apparation" means he'll have a teleport speed.
"Levicorpus" means he'll have forced movement, probably as a minor.
Flight... does what it says on the tin.
The Horcruxes aren't really relevant, as the encounter will assume that the epic level PCs have disposed of them all already. If they haven't, the epic will have an "on death, not really dead" effect.
Transfiguration master (does this list ever end?): not hugely interesting, but will be part of the lawbreaker thing.
Thoughts on the killing curse: Instant death is boring, therefore instant death is bad. The threat of instant death, is good. For this, we'll be stealing the beholder death ray:
5—Death Ray (Necrotic): Ranged 10; +38 vs. Fortitude; 2d6 + 10 necrotic damage, and if the target is bloodied, it is dazed (save ends). First Failed Saving Throw: The target is dazed and weakened (save ends both). Second Failed Saving Throw: The target dies.
We start with the known: we want to challenge a group of 5 30th level PCs with the big bad of an entire campaign. (As an aside, it's quite feasible to flavour the 4e classes such to fit into the potterverse).
The highest level creatures in the compendium are level 36, so Tommy will be 36, himself.
Medium Immortal Humanoid (Human, Undead) for keywords.
Monster builder tells us 1296 HP, 50 AC, 48 fort, 48 reflex, 49 will, and we'll be bumping will high and fort low, becuase that feels right. Dropping fort to 45, and will above AC to 52.
Speed 6, Fly 8 (Hover), Teleport 10 gives us someone who is most annoying in movement speeds and a nice moving fight.
Roughly speaking, he's got 3 "sthicks" beyond the unforgivable curses, which will be his bread and butter.
Fire, Lightning, and Charms.
This gives us 3 "modes" for the combat to progress through, modifying the primary abilities.
(I wish I could give credit, but I can't seem to find the link. Inspired by a recent encounter featuring a blue half-dragon solo with 3 modes. )
We'll start with the "base" monster.
- Magic master: none of Tom's ranged attacks provoke attacks of opportunity, may use ranged basic attacks as melee basic attacks
- Master Duellist: Acts on two initiative counts
- Rennervate: May treat any status effect as a save ends effect, and may save against any negative effects at the start of each of his turns.
- Standard actions:
- (Ranged Basic attack) Killing Curse: 4d12+31 Psychic and Necrotic. If the target is bloodied, it is dazed (save ends). First Failed Saving Throw: The target is dazed and weakened (save ends both). Second Failed Saving Throw: The target dies.
- (Ranged Basic attack) Cruxio: 3d12+19 Psychic. The target takes -5 to all saves, defenses, and attack rolls (save ends all.)
- (Ranged Basic attack) Imperio: 3d8+15 Psychic. The target grants combat advantage (save ends). First failed saving throw, the target is dominated (save ends).
- Mastery of Dark magic: Tom may perform up to 2 charms or unforgivable curses at two different targets.
- Summon minions: If Tom has less than 6 Inferi alive within 6 squares of him, he may summon enough to bring the total up to 6.
- Reflect spells, immediate interrupt, may retarget one ranged or area burst attack.
Summary of base monster: He's nasty, but never takes a party member out of combat without a "warmup" first. This keeps the threat of death and domination high without having party members sit out stunlocked. The -5 to attacks of crucio is effectively stunned, but without the nasty "just roll a die" stun. He's got his signature "I'm awesome at magic" and "I'm awesome at the unforgivable curses" going, with a nice background of "mmm, minions" to keep the fight interesting.
At 1300 HP (roughly), and at 43-141 damage per round from PCs (cite) call it 92 damage per PC per round, 14 PC rounds of combat, not counting the negative status effects. That feels about right, though HP will have to be adjusted to suit party composition.
Now, to add in even more flavour, and to mix up tactics, we'll take a page from here. Functionally, every third of his HP, Tommy will switch flavours, to represent increasing desperation. The swap will be an aura power, a different standard action, and some kind of minor/interrupt.
The first third is the "Charms" school. Charms are "enchantments of objects that cause unusual behaviour"
- Minor action: "Severing charm" Tom causes an item that is not a weapon, implement, armor, or neck slot item to be "damaged" (save ends.) A damaged item is not useful in combat.
- Standard action: "Confusing charm" Target must choose targets randomly for any power they use. (Save ends)
- Aura: Legilimency, range 5, targets must roll twice and take the lower of two rolls when attacking Tom.
This features mainly "annoying" attacks that heighten expectations. When Tom is reduced to 2/3rds hitpoints, he attacks everyone on the battlefield (including his minions) with Electrified Vengeance.
Electrified Vengeance: All creatures in a close burst 15, 15d10+60 lightning and thunder damage.
Forceful curses: All of Tom's curses may slide a creature 4 squares in addition to their other effects.
Aura: Lightning ring: starting 4 squares away from Tom, and extending another 4 squares is a lightning ring, which causes 30 ongoing lightning damage (save ends) to any creature that starts their turn in the aura. And he will keep sliding them into that aura, and teleporting where it will do the most harm.
When Tom is reduced to 1/3rds hitpoints, he then switches to fire. As a free action, he drops the lightning aura and conjures 4 fiendfyres within range 10, area burst 1. Creatures that start their turns within the zone take 50 fire damage. (Ow, it's hot.)
- Minor action: move a fiendfyre zone up to 6 squares.
- Standard action: Gout of flames, Each area of fiendfyre attacks all creatures within 5 squares with 4d12+31 fire damage.
Obviously, the above creature needs to be tested before being thrown as the final monster of an epic campaign. I haven't included a formal stat block because all the numbers will need to be tweaked by the GM to fit the party. This was mainly an exercise in how to create a themed solo that will keep the players interested in 4th edition.