I wonder who it is referring to. "They said" -I wonder who They could be. The Ahamkara? The Nine? The Vanguard? The Ahamkara are my best bet, as it is well known that Guardians sought them out to achieve the impossible. However (if I recall correctly) the deals that were made to receive their gifts always turned out badly. While the premise of a symbiotic creature that consumes darkness, and leaves the host filled with light seems really good, the text has a darker tone to it, doesn't it? Like the next word should be "But". But it isn't what I expected, or but it wasn't worth the price, or whatever. My money is that the symbiote was a blessing given by the Ahamkara. But that's just a guess based on the one canon sentence I can find about it.