Ghaunadaur

Ghaunadaur, also known as The Elder Eye and That Which Lurks, is a greater deity and the god of abominations. This monstrous deity demands that his followers' prayers to him come coupled with the sacrifice of a living being, particularly delighting in creatures that offer themselves to him without resistance. To curry his favor, many of his priests use mind-affecting magics on potential sacrifices in order to ensure such a level of compliance.
There are secret, subterranean altars and cults to The Elder Eye all over Toril, but to have such a powerful worshipper in the middle of Neverwinter, even in its ruined state, is strange. It appears that the Ghaunadaur priest was not attracted by the Blacklake sludge, but that the sludge was transformed by Ghaunadaur's power. It's unclear if the priest was working alone, or if he is part of something larger.