For starters, this critter was more powerful than a pit fiend in nearly every way in BG2. It had 40 more hp, 1 better AC, and better physical stats and intelligence (but lower Wis and Cha).
So here's a rough stat block to begin with:
Demon, Chromatic
Large Outsider (Chaotic, Evil, Extraplanar, Tanar'ri)
Hit Dice: 20d8+220 (310 hp)
Initiative: +9
Speed: 40 ft. (8 squares), fly 90 ft. (good)
Armor Class: 37 (-1 size, +9 Dex, +19 natural), touch 18, flat-footed 28
Base Attack/Grapple: +20/+38
Attack: Slam +33 melee (1d10+14/19-20 plus energy damage)
Full Attack: 4 slams +33 melee (1d10+14/19-20 plus energy damage)
Space/Reach: 10 ft./10 ft.
Special Attacks: Alternate form, spell-like abilities
Special Qualities: Damage reduction 15/cold iron and good, darkvision 60 ft., immunity to poison, spell resistance 29, telepathy 100 ft., variable immunities, variable vulnerabilities
Saves: Fort +23, Ref +21, Will +18
Abilities: Str 39, Dex 29, Con 33, Int 28, Wis 22, Cha 23
Skills: Balance +11, Bluff +29, Concentration +34, Diplomacy +33, Disguise +6 (+8 acting), Hide +28, Intimidate +31, Jump +20, Knowledge (any four) +32, Listen +29, Move Silently +32, Search +32, Sense Motive +29, Spellcraft +32 (+37 scrolls), Spot +29, Survival +6 (+8 following tracks), Tumble +32, Use Magic Device +29 (+31 scrolls) (17 at 23 ranks)
Feats: Empower Spell-Like Ability (acid fog), Empower Spell-Like Ability (chain lightning), Improved Critical (slam), Quicken Spell-Like Ability (acid fog), Quicken Spell-Like Ability (chain lightning), Power Attack, Weapon Focus (slam) [7 total]
Environment: Infinite Layers of the Abyss
Organization: Solitary
Challenge Rating: 21
Treasure: Standard coins; double goods; standard items
Alignment: Always chaotic evil
Advancement: 21–30 HD (Large); 31–60 HD (Huge)
Level Adjustment: -
A vaguely humanoid cloud of vapors, towering over twelve feet tall, crackles with electricity. In seconds, it shifts color to a fiery red, and a palpable heat can be felt near it. It then becomes white-hued and sparkling with tiny ice crystals, the air around it dropping a few degrees. A few seconds later, it is greenish-tinged and gives off an acrid smell. Throughout its shifting forms, its eyes always remain dark pits of steadfast malevolence.
Although individually a match for even a balor, chromatic demons rarely have political aspirations in the Abyss. They relish destructions, and often serve as guardians for powerful demons and other fiends for opportunites to slaughter infiltrators.
A chromatic demon stands about 13 feet tall, but weighs only 20 pounds due to its vaporous nature.
Chromatic demons speak Abyssal, Celestial, and Draconic.
Combat
A chromatic demon typically opens combat in its cloudy air form, releasing a quickened empowered chain lightning to affect as many opponents as possible while striking with its electricity-tinged slams. It uses its Knowledge skills to ascertain opponents' vulnerabilities, and shifts to a form that takes advantage of these weaknesses. If it can't determine vulnerabilities through skills, it shifts forms until it finds one that seems successful. If the opposition use protective spells, it liberally uses greater dispel magic to reduce these wards.
A chromatic demon's natural weapons, as well as any weapons it wields, are treated as chaotic-aligned and evil-aligned for the purpose of overcoming damage reduction.
Alternate Form (Su): A chromatic demon can shift between its four forms as a standard action. It gains different energy damage, immunities, and vulnerabilities in each form.
In cloudy air form, the chromatic demon gains immunity to electricity and vulnerability to acid. Its slam attacks deal an additional 2d6 points of electricity damage.
In fiery form, the chromatic demon gains immunity to fire and vulnerability to cold. Its slam attacks deal an additional 2d6 points of fire damage.
In icy form, the chromatic demon gains immunity to cold and vulnerability to fire. Its slam attacks deal an additional 2d6 points of cold damage.
In corrosive cloud form, the chromatic demon gains immunity to acid and vulnerability to electricity. Its slam attacks deal an additional 2d6 points of acid damage.
A chromatic demon remains in one form until it chooses to assume a new one. A change in form cannot be dispelled, nor does the chromatic demon revert to any particular form when killed. A true seeing spell, however, reveals all forms simultaneously.
Spell-Like Abilities: At will—acid fog (DC 22), blasphemy (DC 23), chain lightning (DC 22), greater dispel magic, greater teleport (self plus 50 pounds of objects only), ice storm, power word stun, telekinesis (DC 21), unholy aura (DC 24); 1/day—quickened empowered acid fog (DC 22), quickened empowered chain lightning (DC 22); 1/day—fire storm (DC 24), implosion (DC 25). Caster level 20th. The save DCs are Charisma-based.