The dragon is ready for battle... Its spell selection is mostly based on defensive and emergency exit spells. The few offensive spells don't deal energy damage, but weaken or slow the foes instead.
Sjachixen (very old gargantuan red dragon [fire], CR 21, 751 years), "Fire Shadow"
Initiative: +0
Speed: 40 ft., fly 200 ft. (clumsy)
Senses: darkvision 120 ft, low-light vision x4, listen +30, spot +30
Languages: common, draconic
HD: 31d12, 450 hp
AC: 42, touch 9, flat-footed 42 (-4 size, +30 natural, +3 deflection, +3 armor)
Saves: Fort +25, Ref +17, Will +25
Immunities: sleep, paralysis, fire
Resistances: DR 15/magic, SR 26
BAB +31, grapple +56, trip +25 vs. +29
Full Attack:
1x Bite +40 (4d6+13, 20/x2), 20 ft.
2x Claw +38 (2d8+6, 20/x4), 15 ft.
2x Wing +38 (2d6+6, 20/x2), 15 ft.
1x Tail Slap +38 (2d8+19, 20/x2), 15 ft.
or: Tail Sweep, DC 33, 2d6+19 (only max. small creatures)
or: Crush, DC 33, 4d6+19 (max. medium creatures)
or: Breath Weapon, DC 33, 60 ft. cone of fire (18d10, recharge time 1d4-1 rds)
Abilities: STR 37, DEX 10, CON 27, INT 22, WIS 23, CHA 22
Feats: Multiattack, Flyby Attack, Snatch, Hover, Wingover, Power Attack, Iron Will, Suppress Weakness, Overcome Weakness, Recover Breath, Silent Spell
Skills: Appraise +11, Bluff +18, Concentration +42, Knowledge (arcana) +21, Knowledge (dungeoneering) +11, Knowledge (geography) +11, Knowledge (nature) +11, Listen +30, Move Silently +25, Search +30, Sense Motive +26, Spellcraft +30, Spot +30, Survival +21, Use Magic Device +28
Special Qualities: no cold weakness, blindsense 60 ft.
Spell-like Abilities: 9/day - Locate Object, 3/day - Suggestion
Spells known (CL 13):
0: Dancing Lights, Detect Magic, Flare, Ghost Sound, Guidance, Mage Hand, Prestidigitation, Read Magic, Resistance
1: Alarm, Disguise Self, Magic Missile, Ray of Enfeeblement, Shield
2: Cure Moderate Wounds, Invisibility, Resist Energy, See Invisibility, Silence
3: Blinding Breath, Dispel Magic, Haste, Protection from Energy
4: Dimension Door, Evard's Black Tentacles, Greater Invisibility, Stoneskin
5: Cloudkill, Righteous Might, Transmute Rock to Mud
6: Antimagic Field, Word of Recall
Used equipment: Ring of Protection +3, Bracers of Armor +3, Claws of the Ripper, Amulet of Proof against Detection
Plan:
The dragon has placed three alarm spells (mental alarm) in the cavern that the PCs will pass through. The cavern was used by the cultists' orc guards before (until they got killed), and the dragon has hidden a few coins at some sleeping places of the orcs. The PCs haven't searched the area yet, and they do not stay there when the cultists and the dragon set up their ambush.
When the first alarm goes off, the dragon knows where the PCs are, and he will start to cast his long-term defensive spells: Resist Energy (acid), Protection from Energy (cold), Stoneskin (in that order). When the second alarm goes off, he casts See Invisibility and Disguise Self. When the final alarm goes off, he casts Shield and Invisibility.
The cultists get the information about the triggered alarms by signs. They have Darkvision activated since nightfall and will hide behind an Illusory Wall above and to the side of the cavern.
As soon as the PCs leave the cavern at night, the dragon will cast a silenced Greater Invisibility. The PCs will see a dark landscape weakly illuminated in red from the fire in the volcanic crater, and ash is falling out of clouds lingering low in the sky. The erupting volcano is making a lot of noise: hissing sounds, explosions, rocks cracking. The ground is trembling slightly. Overall, it is a very dark, eerie scenery.
Once the party is in the open area and the cultists' remaining guards start their ambush using bows, the dragon casts Evard's Black Tentacles to hinder a possible retreat of the PCs. The cultists get ready to interfere with spells cast by the PCs. Because it is a surprise round, the evil guys only have standard actions. The cultists don't attack because their part in the ambush is to block all exit spells of the PCs. They can't fulfill this task if they get detected early and are forced into a spell duel (which they won't survive for more than 1 or 2 rounds).
In the first regular combat round, the dragon will cast Blinding Breath and dive-fly-by-attack the group with his breath weapon. This will do about 100 damage at a decent DC, half of the group will likely make their save (the two brass dragons will ignore the fire, but they are seemingly no big threat

), while the others will now be burned and blind. I assume that the party is protected by Resist Energy (fire), and the damage will be 70 (or 20 if saved) instead. Not that much for level 25 guys, but the blindness is nasty and the dragon is still invisible. The bowmen will fire as long as possible, their arrows are non-magical, thus they are not afraid of hurting the dragon (but the dragon will not take care of their healthy anyway).
Depending on the PCs actions, the cultists' cleric will disrupt the first PC exit spell by casting Silence, while the necromancer and the wizard readies to cast a Dimensional Anchor if any susbsequent teleportation spell is cast. The dragon will cast a Ray of Enfeeblement on the strongest-looking PC and then position himself for melee combat against the party wizard or cleric (depending on their actions and if not blinded, he'll ignore weak and/or blinded PCs if possible). If his breath weapon was not as successful as expected, the dragon will cast AMF instead of RoE and position himself close to the wiz or the clr...
From then on, it is difficult to foretell how everything develops.
The cavern will collapse due to the eruption during round 2, and spit forth smoke, ash and some lava bombs in the process.
I decided not to go straight with tha AMF but use it as a back-up if the PCs think that they can handle the situation all too easy (or if they were too lucky with their saves).
Two things add a chaotic element to the battle:
1) The artefact that is in the possession of the cleric (or the cleric is possessed of the artefact

). It will work even in an AMF, and could force the dragon to retreat ASAP (or be dead).
2) The mnk/ftr/drd is infected by lycanthropy (totally forgot that), and it is now her "first" full moon... I intend to play it out a bit similar as in that Van Helsing movie, where the werewolf was in wolf form only when in moonlight. The PC will change shape when the moon is visible between the clouds, and this would happen at the right moment.
Comments or alternative strategies are welcome!
The Encounter EL&XP calculator 4.01 lists that as a trivial challenge. However, I increased the dragon's CR from 21 to 25 because of his extensive preparation and AMF option, which makes it a challenge (EL 27 would mean very difficult).