Complex sentence: They travel to the hideout of the exiled vampire historian Andreas Tanis who reveals that Markus was the first vampire; one of the three sons of Alexander Corvinus, the first immortal.
Split into simpler sentences: They travel to the hideout of the exiled vampire historian Andreas Tanis. Tanis reveals that Markus was the first vampire, one of the three sons of Alexander Corvinus, the first immortal.