Help me with the CR for this encounter? -- my players out!

[Kunimatyu]

I've cooked up a little something special for a one-shot, but I have no idea what the CR for each of these guys are, since I've used bits of templates, rearranged or removed extra natural attacks, and given the ogre several unique magical items. My party will be around level 8, and they'll have just finished fighting a CR9 Aspect of Demogorgon before this lovely pair shows up. Are they too tough for a group of 4-6 level 8s? Not tough enough?

Kelathrax, Zombified Young Adult White Dragon
HP:15d12 (100hp)
Init: -1
Speed: 60ft, cannot run
AC: 25 (-1 size, -1 Dex, +17 natural), T8 FF26
Space/Reach: 10ft/5ft (10ft with bite)
BAB/Grapple: +15/+25
Attack: Bite +20 (3d6+9) or bite +20 (3d6+6) and tail slap +15 (2d6+9)
Saves: Fort: +9 Reflex: +8 Will: +9
Breath Weapon: 40ft cone of 5d6 cold (DC 20 Reflex half) every 1d4 rounds
Frightful Presence(Will DC15), blindsense 60ft, DR5/magic, SR 16, immune cold, vuln fire,
fog cloud 3/day(20ft)
Abilities: Str23 Dex8 Con- Int4 Wis11 Cha6
Feats: (6) Power Attack, Imp Bull Rush, Imp Nat Attack x2(tail, bite)
Shock Wave: full-round action, bull rush attempt(+14) 75ft radius, failure = prone
Awesome Blow: full-round action, +16 tail slap (2d6+9), Ref DC = damage or go flying 10ft

Arghosh, the Chosen of Orcus
Ogre Half-Fiend Fighter 2
Init +2
Space/Reach: 10ft/10ft
AC 23, touch 11, flat 21 (AC28 with expertise)
HP 64 (6HD); DR 5/magic
Saves: Fort +12 Ref +3 Will +2
Speed: 30 ft. (6 squares), Fly 30 ft. (Average)
Base Atk +5 Grp +15
Attack: Falchion +11 (2d6+10/18-20) (or at -5 with expertise)
SLAs: (CL 6) 1/day - unholy blight (Will DC 19), clutch of orcus (Will DC 18)
Smite Good 1/day (+5 attack/+6 damage vs. good)
Abilities Str22, Dex15, Con20, Int13, Wis13, Cha20
SQ: DR 5/magic, SR 16, immune poison, resist acid 10, cold 10, elec 10 and fire 10
Feats: (6) Combat Reflexes, Combat Expertise, Thrall to Demon,
Chosen of Orcus(grants clutch of orcus as Cha-based SLA 1/day)
Parrying Retort(when expertising for max, first miss by an opponent = attack op)
Skills: Intimidate +13, Listen+8, Spot +8
Possessions +1 halfplate, +1 whirling falchion(casts whirling blade 3/day), skull of cure critical wounds(as the potion, but crush the skull to gain effect), cloak of charisma +4, boots of stable footing(grant +20 circumstance bonusto resist trip/bullrush, +30 total)

 

[Shazman]

I'm think that the zombified dragon is a CR5? I'm not quite sure since I don't have the all dragon CR's memorized, but I think the CR for zombified dragon's is half the original dragon's CR +1. The half-fiend ogre is a CR7. So I'd say that the encounter level for the two combined is 8 or 9.

 

[Kunimatyu]

The dragon's a bit trickier, as it's not just a Zombie template from the MM, but contains some aspects of the Dread Zombie template from Green Ronin's Advanced Bestiary(phenomeonal book, by the way, if any of the authors are reading this!). A normal Zombie White Dragon is CR5 like you said, a full Dread Zombie White Dragon of that age would be CR9, but that doesn't take into account the loss of his 2 claw and 2 wing attacks, nor the loss of 50some hitpoints from the living version.

The Ogre should be close to CR 7, since he's a CR3 Ogre + 2 levels of Fighter + Half-Fiend template, which is CR+2, I believe. However, he's got two powerful magic items, the falchion that he can use to cast Whirling Blade 3 time per day, and the boots of stable footing, which grant him virtual immunity to the dragon's shockwave attack, which would otherwise often knock him prone.

 

[Obscure]

I'd peg both monsters at a strong CR 7 or weak CR 8. Together, you're looking at an EL 9 or 10 encounter -- just tough enough to be pretty dangerous for an 8th level party that has already used some resources. Should make a good encounter.