The siren star was a star that emitted unusual pulses of electromagnetic energy that interfered with the control systems of Leviathans, forcing them to fly directly towards the star whether they wished to or not. According to Mu-Quillus, who controlled the star, it had claimed the lives of 83 Leviathans at the time the ship Talyn was caught by its power. Being only half Leviathan, Talyn was able to temporarily resist the pull of the star with the help of his shields, but even he was not immune.
The siren star was used by Mu-Quillus to attract and kill Leviathans according to his contract with the Patriqu-ro, a race of shipbuilders who saw Leviathans as competition and wished to destroy them. He was somehow capable of changing the frequency and consistency of the electromagnetic pulses, muffling or intensifying them as he wished. There seemed to be some kind of point in the star where the pulse were generated from, which Mu-Quillus had somehow linked himself to, as he died when Talyn destroyed this point with a blast from his cannon. ("Meltdown")