Question: Are the Old Ones considered gods with clerical domains? I would think not.
We could skin the Seven Sisters Three as the arcana domain gods. This would lend itself to the tri-wizard creation myth.
Either way, the last domain unfilled is the Arcana domain unless we want to make more domains or see what unearthed arcana has to offer.
Susej is worshiped in the form of his corporeal avatar which was said to have visited the material plane some millenia ago. (1) His aspect caused much strife though he himself proclaimed a message of peace (1). As a result of the cognitive dissonance, the rulers of the land he was in subjected him to a cruel death on the wheel which was then hung on a pole for all to see (1). His most devout worshipers will wear miniature replicas of the wheel and pole and are known to say "the wheel of war always turns." (1)