undetermined (see below)
Tabak had childer who were from noble families of medieval Hungary and Romania. They were selected from a neonate pool of powerful young aristocrats raised specifically to become undead. This ensured the Tzimisce clan elders a line of noble-born fledglings, bred to wield power. During the Anarch Revolt, just after the Lasombra Antediluvian was crushed, Lugoj's army of anarchs invaded Tabak's domain; Tabak managed to escape, leaving his childer to meet their Final Death in his place. The Anarchs killed all of Tabak's childer, less one: Lambach Ruthven, a craven and weak-willed specimen, who they captured and recruited to their cause. Tabak and Lambach would later work together under the Anarch cause, with Lambach answering directly to his sire once more.
At the end of the Anarch Revolt, during the rule of the mortal prince Vlad Țepeș, also known as Dracula, Tabak discovered through one of Lambach's spies that Țepeș was drinking vampiric blood to gain the strength and longevity of a ghoul. However, he had no Cainite master and was instead drawing vitae from one of Lambach's captive spies. Due to this violation of vampiric authority, Tabak led a group of Tzimisce (including Lambach) to depose of the "Impaler Prince". On their way to his fortress they were ambushed by Camarilla Justicars and a group of Archons who slaughtered most of the Sabbat pack, except for Lambach and Tabak.
Before the Justicars could get to their Tzimisce foes, Vlad Țepeș attacked and slew the Archons who were weak from the battle with the Sabbat, then staked the pair of Tzimisce survivors and took them into his custody. After that fortuitous episode, Vlad "persuaded" Lambach to turn him into a vampire. Immediately thereafter, he diablerized the defenseless Tabak.
Tabak's exact generation is unclear. Most sources, beginning with Children of the Inquisition, agree that he was the sire of Lambach Ruthven – and if this claim is true, we could assume Tabak was a 4th generation by examining Dracula's character sheet in 1710 (centuries after the diablerie), where he is presented as a 5th generation. The book Lair of the Hidden gives the lineage of Danika Ruthven as the childe of Damek Ruthven, childe of Ruthven, childe of the Eldest. The Dark Ages Clan Novel 13: Tzimisce also has a character called Damek Ruthven, who is confirmed as the sire of Tabak Ruthven, and if that source would be taken seriously, it would be unlikely that Tabak was a vampire of the 4th generation, let alone Lord Ruthven himself.
In the Encyclopaedia Vampirica, Lambach is described as a 7th generation vampire, sired by Tabak (that in his turn would be a 6th generation). However, it should be noted that the Encyclopedia is written in-character and contains a number of other errors or inconsistencies. Variously then, Tabak could be taken as either 4th, 5th, or 6th generation by the time of Lambach's Embrace – although either or both of them could have subsequently lowered their generation via diablerie, just as Dracula supposedly lowered his generation to 5th by devouring Tabak's heart's blood.
One in-character and one out-of-character accounts presented by Clanbook: Tzimisce Revised states that Lambach is a direct childe of the Eldest, contradicting all other sources that have covered Lambach, Tabak, or both. As a result, some believe that Lambach is in fact Tabak's elder (possibly his sire), and that he had been manipulating the events, trying to look naive and harmless, while being the true mastermind (or at least the major tool of the Antediluvian), behind the First Vaulderie, the Anarch Revolt, the foundation of the Sabbat, and the false diablerie of the Tzimisce Antediluvian.