Alrighty. My foremost idea is a bit out there, so I'll try to explain it to the best of my ability.

The war god of the previous, overthrown pantheon was known by most as a God of Blood. He was a raving, half-mad berserker type (think "BLOOD FOR THE BLOOD GOD") that demanded human sacrifices from his worshippers. Very Aztec-esque. He's so reviled by the other gods, that they do their best to thwart his attempts at having children. The one child that he does manage to sire with a mortal woman is then cursed (hopefully by a willing Sun God) so that he cannot stand the light of day, shares his father's bloodthirst in a far more literal sense, and any of his blood share this curse. He is, in effect, the first vampire. This is my character.

He spends his nights as a demigod scheming to take his revenge on the gods and building a stable population of vampires in the mortal world. Not only do "those of his blood" share in his curse, but they also inherit a small portion of his divine power. After centuries his plans come to fruition, and he stages a grand sacrifice to summon his father, who he then traps. He then consumes his father, down to his very soul, thus elevating himself to full divinity.

This more or less "cures" his curse, but he has inherited his father's need for blood offerings. To accommodate this in a more civilized age, he has taken patronage of medicine, and receives more than enough blood offered in his name from its many misguided practices. He would also be patron of such things as parasites, betrayers, bastards, pariahs, and orphans. He's just as hated by the other gods as his father was, but there is also a considerably element of fear, as he still retains his ability to cannibalize other gods.

Tl;dr: Cannibal vampire god of medicine and outcasts, son of bloodthirsty war god.