“The Ancients are connected to the human world at its uppermost levels. They assume the initiate’s wealth, which helps them insulate themselves and assert their influence across the globe.” - Abraham Setrakian, The Fall
The Ancients are seven original vampires, three of the Old World and four of the New World, their origins are not clear but as the first of the Strigoi, they possess powers that their descendants lack, such as an ability to transfer their consciousness to another vessel.
It is evident that the Ancients are comprised of (to paraphrase Eichhorst) "the white and the worms" infesting their victims, which are not mere pathogens. This invasive characteristic extends their presence and influence outwards from a central host to an indefinite number of victims. On the televised series, hosts of the Ancients are distinguished by possessing a crimson worm signifying their status and essence, as discussed in the Season 3 episode entitled, "Gone But Not Forgotten." As such, they are more like a sentient substance than the body they may be mistaken for when possessing a host. Their vassals are their vessels. (In the Season 2 premiere, "BK, NY," the Master explicitly distinguishes between himself and his host when he instructs his acolyte Eichorst to prepare for the Master's succession because the current body was dying.) This distributed form of existence makes it difficult to contain and eradicate a rogue Ancient.
The Master, the 7th and youngest strigoi, went rogue. when the other three crossed over during the colonization of the New World, he did not accompany them on the voyage. Instead, waiting centuries until his takeover plan was ripe. The 7th going rogue is certainly the reason for this "vampire war."
While the 7th gives eternal life seemingly indiscriminately when he needs nourishment, the six other Ancients, who wish to remain hidden, will kill immediately after feeding, in order to prevent any spread of their virus. The other Ancients view eternal life as a great gift and only selectively give it. One must have power, access, and be obedient in order for The Ancients, operating as the equivalent of the Illuminati in their world, to consider someone as a candidate for immortality.
During the outbreak
During the events of the New York incident, only three Ancients are shown and they favor no direct interaction leaving that task to their minion strigoi who are very loyal to them. They do not spawn mindless strigoi that roam around without any consciousness like the Master prefers to. Yet it is not fully clear whether the process of strigoi turning leaves them open to do that or whether the Master is the only ancient who is able to create Strigoi and keep them enthralled to himself. There has been a very long enduring feud between the Ancients and even Quinlan knew about that for a long time through the TV series does not elaborate when that difference in opinion among them arose.
Although the Master belongs to the New World Ancients, he was most active in Europe as evidenced by the creation of Quinlan who's existence was an accident of sorts. The master fed on a pregnant woman and she managed to escape.
It is unclear whether Quinlan's mother became fully strigoi and gave birth to Quinlan, or whether the pregnancy was accelerated by the worms.
The Ancients keep many secrets about their species to themselves for security reasons. The knowledge of the crimson worm was unknown even to Quinlan who thought he finally destroyed the Master by beheading Gabriel Bolivar whose body the Master occupied for a short time. Abraham Setrakian watched a red worm leaving Bolivar's body and dropping into the sewers where he found another short-time host in a member of the Seal team that was dispatched to hunt him down(this failed since the Seal Team never had a chance against the Master and they were never aware of the Master's array of supernatural powers)
The Master obliterated the Ancients by sending a huge army of strigoi cannon fodder into the sewers and Eichhorst who detonated a time-delayed small nuke weapon which eliminated any threat the Ancients could have posed against the Master.
Judging by their actions the Ancients rather wanted to thwart the Master's plan than fully destroy him, as they all were beings somehow connected to each other. The Master's ability to veil his thoughts from the other ancients was a necessary and crucial part of his plan to seize the world but the Ancients were quite lazy and inactive in their reactions towards the Master and paid for this naivety with their lives.
The Ancients share many traits with the Master, namely being physically more powerful than lesser Strigoi and presumably could communicate and see through those they turned. Unlike the Master, the ancients don't seem to switch bodies often if at all, evidence by their extremely aged appearance. They also seem content to sleep constantly, only awakening if the need arises.
Differences from the books
Unlike their counterparts from the books, The Ancients didn't seem to have any ambition to take over humanity and were rather content to stay in the shadows and feed, which put them at odds with the Master who wanted to leave the shadows and rule humanity. In the books, the dispute is seemingly caused by a disagreement over methods, the Master lacking the patience of the Ancients, who wanted to wait for the right time to take over.
- The crimson worm of the Ancients, mentioned on the televised series, makes allusion to the corrupting worm in a crimson bed in William Blake's poem, "The Sick Rose" (also referenced in the Benjamin Britten Serenade and in the song "Love's Secret Domain" by the band Coil).