We have the first monster we want to show off for Liam's book "Here There Be Dragons". This one has roots in ancient myths, and also something we have wanted to use for a while.
BTW we are not 100% set on what version of the game system we want to use. I opted for Basic tonight since I have been in a Basic mood. Liam wants 3.5.
I am releasing the following text as Open under the Open Gaming License.
Aži Dahāka, the Demon Dragon is copyright 2011, Liam Brannan
Armor Class: -5 
Hit Dice: 22d8+21** (120 hp)
No. of Attacks: 3 bites or Breath Weapon or Claws
Damage: 3d6 / 3d6 / 3d6
Special: Claw Attack, Breath weapons, Poison
Movement: 60 ft. / Fly 120 ft.
No. Appearing: 1 (Unique)
Saves As: Fighter 24
Aži Dahāka is believed to be the offspring of the Mother of All Dragons and the Prince of Demons. Certainly his heritage is evident in his form. He has three dragon heads, each of red, blue and black dragons. He has a long black, serpent like body with two tails and four wings.
Aži Dahāka is a unique creature and has been worshiped as a god in the past. He has divine parentage but is wholly a mortal beast and can be slain, though with difficulty.
Aži Dahāka is not the smartest of dragon- or even demon-kind, he is however a very clever combatant and knows how to take advantage of many situations. If he is out numbered he will attempt to reduce the opposition with his breath weapons.
Bite: The primary attack of Aži Dahāka is his massive bite. He may attack with all three heads per round on separate targets. On a roll of a natural 20 Aži Dahāka bites the victim whole and swallows.
Aži Dahāka has a second set of teeth in his throat that can automatically do 1d8 points of damage per round unless the victim is freed.
The bite of Aži Dahāka also includes a very deadly poison. Each successful hit that causes at least 1 point of damage will require the victim to make a saving through vs. Poison.
Breath Weapons: Aži Dahāka as appropriate has a breath weapon for each head. Though the chaotic nature of his birth has gifted the black head emits an electrical blast, the blue head breathes fire and the red one spits acid. Each head will do 2d10 (2-20) hitpoints of damage per attack. Aži Dahāka may attack with two heads simultaneously per round.
Claw Attack: Aži Dahāka can rear up on his hind legs to make a claw based attack. This is the only attack he can make in that round. He can rise up and attack with both claws. The attacks do 1d10 points of damage each and can hit creatures normally immune to normal attacks, i.e. requiring magic to hit.
Aži Dahāka is a strong fighter, but knows the value of retreat if the combat is going against him. If forced to retreat, he can alter his skin color like that of a octopus and blend into the background. This gives him 95% invisibility. Since this is a natural effect, See Invisible or Detect Magic will not locate him.