Taranis is the supreme Dark God. He is also one of the New Gods, who drove out the Elemental Lords during the Age of Catastrophe. (The Elemental Lords have been driven out, but they still have adherents, and obviously, their magicks are still studied, though in a much less prominent way at places like Sistarra. The Elemental Lords have, at various times in Etheria's past, attempted to regain their former glory...)
Adramelech is not a god, though he is quite powerful in his own right. He is the ruler of Infernia, another Realm of existence, one from which the demons are spawned. While in Infernia, he projects terrible power and control over his subjects -- in this respect, he is virtually like a God in his own Realm. Dark Mages in the realm of Etheria can summon an aspect of him into the arena, and though a deadly threat, he is far from his true deadly self. (The same goes for Malacoda).
As a God, Taranis projects his power to both Infernia and Etheria. Adramalech (and all the demons) fear and follow him.