Tiamat is a main character in the 1983 Marvel Productions Ltd., D&D Enterprises and TSR nonlinear animated series, Dungeons & Dragons. Tiamat is almost certainly voice acted by Frank Welker, who did many of the series' incidental voices.
When six children are thrown into an alternate world called the Realm the first thing they see is Tiamat. A towering five-headed female dragon, a hydra, Tiamat bumps into the children while trying to catch an early morning meal - Uni, the unicorn. When the children attack Tiamat with their new magical weapons they make a dangerous enemy who will not hastily forgive them.
Tiamat's five heads divide this way: A red-dragon head that fires fire. A blue-dragon head that fires lightning. A green-dragon head that fires poison gas. A white-dragon head that fires cold. A black-dragon head that doesn't seem to fire much of anything, and so is probably just there for the bite.
As we learn during the series, Tiamat is the most powerful creature in the Realm. Indeed, it is its presence that stops Venger and his dark armies from total domination. Almost no weapon will pierce Tiamat's scaly red-dragon hide, and her five heads, each with a different attack - fire, lightning, poison gas, cold and bite damage - keep any foes at a safe distance. As well as these formidable qualities Tiamat is also a high-level spellcaster, as we see in The Dragon's Graveyard where it conjurs a portal that leads out of the Realm.
Even within this nonlinear series, the ostensible first episode features Tiamat as the children's first foe. As they make their way across the Realm's hazardous wastelands, Tiamat follows their movements - always ready to interrupt where it sees benefit. As the series wears on Tiamat's visits became less and less frequent and its last appearence in the series is in the second season in The Dragon's Graveyard.
With its lisping manner and tendency to jump out of the shadows it is easy to think Tiamat lacks purpose entirely. However, in The Dragon's Graveyard Tiamat seems overjoyed to help the children, if it will only mean the end of Venger. Indeed, Venger wouldn't be after the children at all were it not for Tiamat, since he requires the children's magical weapons to have any hope of defeating it. Only once does Venger get the opportunity to face Tiamat with the weapons, in The Hall of Bones, and there the weapons are without power and virtually useless.
It has been said that Tiamat, more than any other character, typified the Dungeons & Dragons cartoon series. Of all the many personalities, quests and riddles it is the visage of the lisping hydra that persists in the public mind. Maybe then, even if Tiamat had only a small contribution to make to the series plotwise, it played a much larger part in the series' popularity.