The history of the Dawnguard is vague and surrounded by rumors and hearsay. What is known however is the group was formed in thehttp://elderscrolls.wikia.com/wiki/Second_Era by the http://elderscrolls.wikia.com/wiki/Jarl of http://elderscrolls.wikia.com/wiki/Riften_(City). The Jarl's son, through his overly confident and adventurous pursuits, contracted http://elderscrolls.wikia.com/wiki/Vampirism. The Jarl refused to kill his own son, so instead commissioned the construction of a fort and contracted a large mercenary force, branded the "Dawnguard", in order to keep his son quarantined.
Some say the Dawnguard had to put the Jarl's son down and were banished from the hold. The Dawnguard however continued their work as vampire hunters, until they broke up, due to the fact people increasingly disregarded the threat of vampires, weakening the power of the Dawnguard.^{http://elderscrolls.wikia.com/wiki/Dawnguard_(Faction)#cite_note-0}
Others claim that the Dawnguard, upon seeing the immense power of a vampire, decided to become vampires themselves. They were said to have preyed on the Rift until they were hunted down.^{http://elderscrolls.wikia.com/wiki/Dawnguard_(Faction)#cite_note-1}
Rumors say that the Dawnguard were disliked because of their abuse of power as they went on nightly raids and pillaged small villages without any probable cause.
-So who agreed about dawnguard should exist in TESO and with same fortress we see in TES skyrim and we can join dawnguard for vampire hunt-

That would actually be ridiculously awesome.