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!
Best Answer
You can absolutely use their stat blocks directly as allies. The real question you have is how to make the ensuing encounter appropriately challenging with the additional allies. You have a few ways to handle this.
Increase the encounter to make up for the allies
This takes some tinkering, but the way I would start is take the encounter xp of the additional guards, and create an encounter worth the same xp, assigning monsters from that group to handle the guards. This will make the encounter seem larger, but not actually change what the PC's are handling. By the time the additional mobs take out the guards, they will be weakened, or the PCs will have had the opportunity to take out the primary enemies (or both). You can also fudge it by subtracting the guard's encounter xp value from the original encounter to determine how "hard" of an encounter it actually is.
Don't use stats, just tell a story
The better option from my standpoint is to not bother with tracking hp and stats from the guards, and use them for a story point. If you want one of the enemies to seem tougher than the rest, have him catch a guard's blade in his bare hand and kill him in one blow. If the PC's are hurting, have the guards finish off one of the weaker foes, or take a hit for them. You can still roll for the guards, but use their deaths and kills to make a cinematic point. You can probably come up with a lot of uses for the guards in this context without worrying about their stats.
Do a mix of both
Have the guards statted out, but don't have a fixed encounter size. If the guards start "winning" too much, bring in reinforcements from the assassins. If the players are losing badly, have the guards come to the rescue (or at least take out or distract a couple enemies).
Give the guards a different objective
The guards really don't care about the PC's survival, they care about the King. Maybe they are just fighting to get a clear path to get him to safety. Have them leave the encounter as quickly as possible with the King in tow (or maybe some of the guards are in on the assassination attempt).
There are a lot of options, so don't feel constrained by the rules. You are the DM after all, your job is to make an interesting story/game with the rest of the players and adjudicate rules. Sometimes eyeballing it or making things happen because the story needs it is your job.