Rahab's soul was the soul of the Sarafan commander and vampire lieutenant Rahab. Raziel killed Rahab and devoured his soul, earning the ability to swim. The soul appeared in Legacy of Kain: Soul Reaver and Soul Reaver 2.
In the era following Blood Omen: Legacy of Kain, Kain established his empire by recruiting six lieutenants. Kain, raised by necromancy, could not "turn" a vampire via the traditional blood gift. By breathing a small portion of his vampire soul into a corpse, he could draw the corpse's soul back into the body, reviving the corpse as a fledgling vampire.
In an act of calculated blasphemy, Kain raided the tomb of the six martyred Sarafan commanders. Their well-entombed corpses – though long dead – had been spared the corruption of the grave. Going from crypt to crypt, he breathed his reviving gift into each of the Sarafan corpses in turn, and thus raised his six first-born "sons" as the lieutenants of his fledgling empire.