[RPG] I need a formula for calculating the CR for a custom monster

Well, Complete Mage has Fey Presence:

Fey Presence

[Heritage]

You share your ancestors' knack for playing tricks on the minds of others.

Prerequisite

Fey Heritage, character level 6th, Nonlawful alignment

Benefit

You gain the following spell-like abilities, each usable once per day: charm monster, deep slumber, and disguise self. Your caster level equals your character level.

Obvious problems include weird fey influence on your devil, and alignment issues. Both things I’m inclined to hand-wave (for players as well as NPCs), but they are there. Also requires Fey Heritage, which is basically garbage (+3 Will vs. Enchantments). I’ll keep looking.

Aberrant Dragonmark (Player’s Guide to Eberron) works for certain (Humanoid) races but not for an Osyluth (or much of anything else that qualifies for Fiend of Corruption, though I suppose a Half-fiend Human/Dwarf/Elf/Halfling/Orc could do it).

I’m pretty sure that’s it for feats.

For templates, Half-fey (Fiend Folio, same as Fiend of Corruption in the first place) is CR +1, gives charm person at will, and a bunch of other stuff that’s pretty useful (like flight). Of course, if you’re going for something that’ll increase CR, Beguiler (Player’s Handbook II), Dragonfire Adept (Dragon Magic), Sorcerer, Warlock (Complete Arcane), and Wizard all work, and I think Binder (Tome of Magic) does too.

There actually is a level 10 Solo named The Risen King, in Fortress of the Yuan'ti pg 16, but he's a controller and poison/snake themed, which doesn't fit the Leoric concept very well.

Malachi's Butcher, from Dungeon Mag #163 pg 83, brings a different iconic Diablo undead boss to mind, so is also mostly un-helpful here, but he gave me an idea, so let's walk through the process of stealing his Spawn Meat Mote power, and applying it to a level-reduced Kas of Mondahan, who is originally printed in Dungeon Mag #170 pg 55, and was reprinted as Kas the Betrayer in Open Grave pg 204 with some math tweaks that don't really come into play after we de-level him from 26 to 11.

Kas has the following things going for him:

• A Bastard Sword
• A cleave power
• Regeneration
• The soldier template, which gives him high HP and high AC.
• A movement power, which we'll convert from flying to teleportation
• A healing-on-hit power, which we'll allow him to recharge.

I'm removing his dominate, because it's decidedly un-Leoric, and in its place I'm putting in that Spawn Meat Mote power I stole from the Butcher, only spawning a Skeleton Soldier (Open Grave: SotU pg 77) instead of a Meat Mote (they're both minions. Feel free to level up the Soldiers a bit, as the level 6 versions defenses are a tad low -- but Leoric's minions are cannon fodder, easily replaced and highly expendable, so I don't mind this at all.) A slightly stronger option would be Skeletal Legionaries from the Monster Vault, pg 257. These are level 7 minion soldiers.

I am also removing his fire resistance, climb speed, and mist-form, changing his training in Arcana and Stealth to Athletics and History, and giving him immunity to Daze and Dominate. Some of these are to more strictly adhere to Leoric canon, while those last two immunities are more because Solos desperately need all the help the can get in keeping clear of status effects.

I'm changing his mark-punishment from dealing ongoing necrotic damage to another means to summon a minion. In-play, this should work out fairly similarly to the ongoing damage, but consider changing the duration of the mark itself to save-ends, or removing the "this effect can trigger only 1/round" qualifier, if you want it to be more potent. As is, there's a fairly weak give/take between the two minor action options, with one definitely summoning a minion at the cost of 2% of his health, and the other maybe summoning a minion, without a health cost.

I'm also changing the condition upon which Regeneration is disabled. Sunlight implies that the original was a Vampire. Instead, let's make it stop working for one round after being hit with a Radiant power. This gives the paladin a reason to do the happy dance, and potentially gives the invoker interesting choices to make, given how many minions will be afoot.

After leveling him down to 11 (Level+4), this nets us:

Kas of Monadhan, the Skeleton King
Medium natural humanoid (undead)

Level 11 Solo Soldier
XP 3000
Initiative +8
Senses Perception +10; Darkvision
HP 440; Bloodied 220
Regeneration 20 (regeneration doesn’t function the round after Kas is hit by a Radiant power)
AC 25; Fortitude 26, Reflex 21, Will 26
Immune disease, poison, daze, dominate; Resist 10 necrotic; Vulnerable 10 radiant
Saving Throws +5
Speed 7
Action Points 2
Standard Actions:

Bastard Sword (standard, at-will, MBA) Weapon
+16 vs AC; 1d10+14 damage.

Blood Drain (standard; requires combat advantage against the target, recharge 6) Healing +16 vs Fortitude; 2d10+14 damage, and the target is weakened (save ends), and The Skeleton King regains 50 hit points.

Betrayer's Whirlwind (standard, at-will) Weapon Targets up to three creatures; +16 vs AC; 1d10+14 damage, and The Skeleton King slides the target 3 squares.

Minor Actions:

The Skeleton King's Challenge (minor 1/round, at-will) Necrotic Close burst 5; targets one enemy; the target is marked until the end of The Skeleton King's next turn. If the target makes an attack that doesn’t include The Skeleton King as a target while marked by this power, a Skeleton Soldier appears in a square adjacent to them and immediately attacks them. This effect can trigger only 1/round.

Raise Skeleton (minor, at-will) Effect: the Skeleton King takes 10 damage. A Skeleton Soldier appears in a square of the king's choice within 2 squares. It acts immediately after the king.

Triggered Actions

The Skeleton King's Recovery (free action when kas becomes bloodied) All adverse effects affecting Kas end, Blood Drain recharges, and then The Skeleton King takes a standard action after the triggering action is resolved.

Regal Assault (immediate reaction, when hit by an attack by a target within 5; at-will) Necrotic
The Skeleton King teleports up to 5 squares to a square adjacent to his attacker and immediately attacks them with his Bastard Sword. If it is available, he may use his Blood Drain attack as part of this power. If The Skeleton King is bloodied, a Skeleton Soldier appears in the square The Skeleton King vacated.

Alignment Evil Languages Common
Skills Athletics +15, History +13, Intimidate +16, Religion +13, Endurance +12
Str 21 (+10) Dex 12 (+6) Wis 11 (+5) Con 14 (+7) Int 16 (+8) Cha 23 (+11) Equipment: bastard sword , plate armor .

I quite like this result. His damage isn't too massive, so feel free to tweak it, or give him more minions at the start, or even more HP. But he's pretty tanky between his healing and regeneration, and should be spawning 2-3 easily dispatchable minions/round, which should keep the invoker occupied, and he should almost always be making 2-4 attacks/round.

If your players are decimating him, you can adjust on the fly by deciding not to remove his regen on radiant powers while he's bloodied, or by doubling his Blood Drain heal while bloodied, or by increasing the recharge on blood drain to 5/6 or even 4/6 while bloodied, or similar adjustments.

If your players are having a terrible time (or if your melee characters have attack bonuses under 13 (which would hit him on a 12), consider dropping his AC a few points. Swapping him to a Brute template would be as easy as reducing his AC by 4 and adding 1d10 to all of his damage rolls.

Remember to use his skeletal minions to flank with to gain CA for Blood Drain.

I hope this was a helpful exercise in monster creation, regardless of if you use him!