I was watching a Netflix show (Yu Yu Hakusho) and had an idea.  I wanted a character arc similar to one of the Yokai only a little backwards.  One of the Yokai in the series becomes good due to the love of his surrogate mother.  My idea is a peaceful priest who loses faith and makes a deal to save his child.  Where I need help is how to deal with the change from priest to warlock.

His domain and service are vital to a backstory and can offer up story options for a good DM.  I want the mechanics of losing faith similar to a paladin that becomes an oath breaker.  I don't think he needs to become outright evil but as an Undying Warlock, he will not be able walk the normal path.  Can still have morals, but he literally sold his soul to heal his child.  I picture him as a rural, small town priest.  Low-level, like 2 or 3 when this happens.  If you make the leap at much higher I would expect serious consequences.  Fallout costs him greatly.  His child lives but he sort of dies in its place.   Play story can begin from there.   Would this be reversable?  Could there be redemption for him?  

I am most familiar with Faerun but figure this idea might work in multiple settings.

I see it as a multi-class situation.  Cleric skills change from whatever domain/God he previously served and are now from the Death domain.  His patron would operate as a replacement deity.  Any levels gained after the event are gained as an Undying Warlock.  I like to have a reason for class changes.  It does not make sense for someone to wake up one day and say,  today I will be a monk or whatever min/max situation develops.  Just not sure what the mechanics would look like.  Is it a good story but to difficult for a campaign?

I plan to play around with a character sheet and see if this can make sense.  Any thoughts?

Is it a good story but to difficult for a campaign?

I think it is a good story idea with lots of potential.

BUT

I think this is one of those ideas that you bring to the DM first and discuss whether it works in their setting and how. You are going to need the DM to be bought fully into the story for it to work, though.

Cleric skills change from whatever domain/God he previously served and are now from the Death domain.

As a DM I find it more interesting to have the original cleric skills and domain stay. While the character has turned from their god, the deity has not left them behind and still loves them.

BUT (see how fun that word is?!)

I'd have the deity decline to empower acts that are in direct opposition to the tenets of their faith and domains. That becomes the "Come back to the faith child, we can solve this without the evils you've embraced.

Also, the class change could be as simple as the cleric picking up a cursed object (arcane focus) powered by the will of the warlock patron. That suits the "sudden change" nature of things and the emotional torment of wanting to do good but... being pulled by evil.  Every time the character takes a warlock level and gains new power their good and decency fades just that much more.

Quote from Anhale >>

I picture him as a rural, small town priest.  Low-level, like 2 or 3 when this happens

I'm kind of confused what you're asking here, to be honest

If this is a campaign starting at a higher level, then sure, take a couple levels of cleric, then keep advancing in warlock from there. You already have the backstory -- they sold their soul to save their child, and forsook being a cleric. The "mechanics" would be the same as for any other multiclass. The biggest backstory issue I have with that is... why did they leave home to become an adventurer, after making a sacrifice like that?

If you're saying that the campaign is starting at 1st level though, and you want your character to have been a leveled cleric before resetting as a 1st-level warlock... there's no reason to make it that complicated. Priests in small villages don't need to be leveled clerics, and you can just take the Acolyte background or the like to account for whatever religious training the character had