To be honest, I don't quite know what to do about the husband. Perhaps he could be the entity's voice box? Whenever it has a command for the woman to obey, it is spoken through the husband's mouth (but with some otherworldly voice).
I suppose you could have him assisting the entity, perhaps by tying/chaining the wife to the bed and then slinking away so that his master can be with her... while he watches and jerks off. I don't know if you want to have the entity rewarding him, from time to time, by letting him take the wife anally, or if you want to keep the wife completely pure for the entity. Maybe the entity could reward/appease the husband's urges by somehow luring in other unsuspecting women for the husband to have one night stands with?
Hey! How about this for an idea: You could write the story as if you were the husband. The story itself could be written like it was a gospel for the new cult - recounting the events surrounding the reappearance of the deity, all written by the husband (you) who witnessed it all. That could be his way of serving the entity, as the spreader of the word.
It could give the husband character the chance to express his own arousal directly, by explaining how well the entity screwed his wife and sharing how exciting it was to watch her come loudly every time.
All of these are excellent suggestions, especially the part about the husband tying his wife tot he bed and then jacking off while the entity has its way with his wife. And yes, the story will definately be written from his perspective. However, I doubt the deity would want to reward the husband with one-night stands with other women since it would probably want these other women for itself as well. Maybe he permits the wife to give the husband occassional hand jobs. Do you think that would be enough to keep the husband cooperative and compliant?