Thel 'Vadam was the dragon of the Rider, The General.
'Vadam was originally to be taken to the elves, so they could see whether or not he would hatch for someone there. However, a young General snuck in, and stole the egg, thinking it was a precious stone. As a result, 'Vadam hatched for the boy, and the two became a Rider and dragon.
'Vadam and the General served for many centuries under the commands of their leader. Finally, about a decade before the Fall of the Riders, he and his Rider were promoted to second-in-command to Vrael, and a seat as an Elder in the Council of Riders. He was also one of the Elders who opposed Galbatorix having another egg, despite him being his mentor.
Because he was one of Galbatorix's mentors, he and the General were hunted down and separated from everyone else, a few days before their eventual death. Though Galbatorix had been powered up by magic, he could only barely defeat Thel 'Vadam and his Rider, and he and Shruikan escaped with grievous wounds.
Thel 'Vadam was described as being very kind, yet impatient, and sometimes even grumpy if awoken during a nap. During battle, he is utterly ruthless, tearing apart enemies as if the were pieces of tissue.
'Vadam is enormous, even for a dragon, as a result for being alive for hundreds of years. His scales are described as being "as gold as gold can be". He has a long scar running down his thigh from a fight with a Lethrblaka, and eyes like molten metal.
He was often called "du Evarínya Kuldr", which translates roughly to "the Golden Star" in the Ancient Language.