Ebrietas isn't an alien, though. In fact, none of the Great Ones are--they're actually products of the subconscious, from INNER space. As for Ebrietas, she was likely a product of an experiment. Takes a while to explain, but in short, the Research Hall was the beginning of the experiment, and they used the Kos Parasite and liquid mercury to create Ebrietas in hopes of reviving Kos, but they didn't quite succeed. The true blood they're after is Paleblood, which is related to Mergo and in that vain, to the Pthumerian Queen. Paleblood is the non-physical "blood of the cosmos" that the Healing Church desires. The Queen's blood is what inspired Laurence to found the Healing Church. It wouldn't make sense for the Holy Medium to be Ebrietas' blood, as Byrgenwerth is already in possession of this silvery blood, as seen in Rom and the Slime Scholars. As we all know, Willem was strongly against use of the "old blood" that Laurence brought back with him.