Highlord Bolvar Fordragon was presumably burned by the Red Dragon's flames, but it is untrue. Bolvar's body was salvaged by the Scourge before the flames reached him. His body was dragged to an unknown area, and the Lich King raised him as an Undead, failing to gain control of his mind. The Lich King tortured Bolvar until he finally lost his mind and now serves the Lich King.
Ulduar Event Solved
There is an event in the Yogg-Saron brain room where you can see past events. One of them shows the Lich King talking to a tortured, dark human figure. The figure was indeed Bolvar, and he has been tortured so much, that he serves thew Lich King now.
This is only a fragment of Bolvar's quotes. More coming soon.
Aggro: "Who are you?! Wrynn?! I know of no Wrynn! Saurfang, come! We have uninvited guests who wish to serve!
3/4 Health: "Master! These adventurers! They're mad!
1/2 Health: "You can't beat the master! He... comes for you! He sees dead people!
1/4 Health: "Aaah... the memories... I... I can't fight it!
Player Death: "Ready to serve?!"
Death: "Tell Varian... I'll be seeing him... soon... if he keeps being so... reckless."