In what way? In a non-electronic world? No. Inside a game, yes they are very real.
Death Knights of Krynn happened in 1991.
Death Knights of Krynn was created in 1991.
You can have as many death knights as you want on your account, but you can only have 1 death knight per server (example: you have 5 death knights - 1 is on agamaggan, 1 is on doomhammer, etc.)
No, there are no dragons thus no dragon knights
no
Worgen and Goblin will be Death Knights, because it says it on the site of Blizzard. They just might not be available right away when Cataclysm is released. This is due to Death Knights having a head start in the leveling process.
Yes
Yes, they can.
World of Warcraft: Yes, Death Knights can wear the Bind to Account heirloom armor if they have an 80 on their account with the means to get it.
Technically the Horde and Alliance both formed an alliance with the Death Knights after the Death Knights where betrayed by the Lich King.
sometimes
beacuse they killed him