I'm looking for two suggestions or ideas to get around a little writers block.

Let's say a deity (a) manipulates the chaotic neutral deity of creativity and madness (b) into trapping the neutral deity of fate, destiny, and time in the body of a mortal who is subsequently imprisoned.

My two questions:

1) What is deity (a) the god of?

2) Why would deity (a) do this?

Thanks for taking the time to read.

1. The god is a trickster deity.
2. They did it because either (a) the deity of fate, destiny, etc. was really cramping on their plans to mess with the fates of some mortals elsewhere or (b) trickster deity's gunna trick.

I like Metamongoose's suggestion. Here's another:

1. Deity (a) is a god who was betrayed by their other god friends (or really anyone) and is fated to be unhappy forever
2. The deity is doing this so they can change their fate and get all they've ever wanted. They don't even have to be evil. Maybe they just want to be happy.

Deity of Fate has made a pronouncement (or hinted at) how the first god will be destroyed utterly.  Either in retaliation or in someway to hinder this pronouncement they take time to remove the Fate deity, to hopefully avoid their own fate.

I mean are gods exempt from the powers of Fate?  If one was told that this is how they end would they think they are being targeted by fate unfairly?  Mortals rebel against fate, how much more so would a god rebel against fate.  Might even make them go all Apocalypse on everything because if they are destined to disappear then everything should disappear.  This could be a driving force that changes the gods purpose.  Is this god a "Goodly" god that sees Fates decree that they will be destroyed as an attack against all good in the universe?  This could be a good turning point for the fall of a goodly god from grace and a descent into evil and tyranny, as they see it the only way to preserve their place in the universe.

Even more so if it is an evil god, or one of conquering and law.  Maybe this is time for them to try and take over the entire pantheon, animated Hercules type situation.

Thanks all for the replies, I'll probably put together a mix of the above along with some research I did today. I'll post it here when I do!

Good luck, and hope it plays well.

Deity (a) could be any deity that wants the god of fate out of the way for some reason. A trickster god because he thinks it a hilarious cosmic joke. Or a tryanical deity like Bane who want the god of fate occuped while he seizes control of some sort of destiny controlling artifact. A jealous minor god who seeks to steal secrets of the fates and assume the god of fates power. An ancient dead god who is trying to turn back time to a period where his worshipers are prevalent and be restored to prominence.

So first I want to thank you guys for the great ideas. I've come up with the following based on a mix of suggestions and research. (Mostly suggestions from you kind folks.)

Deity (a) is a neutral deity of Light, Knowledge, and the Sun. They realised with the passage of time and regardless of whether they reached their destiny or fell to their fate they were destined to one day have their twilight and eventual sunset for a new deity to rise and take their place in a new age. Deity (a) then persuades deity (b) to trap the deity of fate, destiny, and time in the newly constructed body of a mortal, who suffers with a madness as a result. 

The mortal doesn't know where they come from, is a mild seer/oracle, but is also housed in a city asylum. They are in every way a real, living breathing person who just happen to also house a deity's essence. A side effect of the entrapment is that most mortals no longer remember the deity of fate etc. The party will have a few interactions with them in unrelated adventures in their capacity as an oracle/seer. Eventually weird happenings such as the day night cycle slowing and stopping, healing magic, medicine, natural healing and injuries/sickness/death no longer having effect, growth stopping etc. will lead them to investigating the mystery of why. So it turns the game into a bit of pure mystery as combat is essentially taken out of the equation for a session or two.

Clues such as texts of the forgotten deity of fate, talking with the odd mortal who remembers them, and a few other things eventually lead them to discover the existence of the trapped deity. The mystery continues with finding them, and finally a moral quandary of how to free them. Killing the host mortal will free them, but that host mortal is also (hopefully) a trusted and valued NPC by this point. Can they find another solution?

I'll probably throw some shenanigans from the deity of creativity and madness, the Sun deity, and others in as well.

It's worth noting that the deities of this particular shared setting my group uses are sort of American Gods inspired in that they are physical beings and it's hard to know whether it was them or mortals who came first in this particular part of the material plane. They don't have stats but they do get involved every so often.

I've got to lay groundwork for this adventure in the current and next adventures and then start into this. I'll update when I've run it and share the adventure for anyone who might like it then. Again thanks for all your help.

Deity A is the God of youth and beauty who seduces deity B into helping them to stop time so everyone remains young and beautiful and continues to worship deity A for eternity.