I am making a dnd game where it takes place after the Battle of Ragnarok, Viking Armageddon. The mortals are struggling to survive now the gods are killed by Surtr and the Frost Giants so it is up to the adventurers to travel to different planes to recover items to rebuild and restore the material plane which is destroyed from Ragnarok. One of the task is to rebuild the sun and moon which was swallowed by Fenris. What items and spells would be useful in recreating the sun and moon? I hope to hear back from you guys. 

You probably want to ask a mod to move this to the General Discussion section or the Story & Lore section. The feedback section here is for feedback on the platform itself.

As for the topic itself, I assume wish is not an option and you want something homebrew but not too homebrew. To rebuild a moon, I would have lots of spell casters cast meteor swarm over and over again at a some point in space; it would probably take hundreds or thousands of years though. For the sun, I would have lots Graviturgy wizards using their Event Horizon class feature to collapse a ball of gas in space to kick start nuclear fusion. There are also Mythallars if you want a huge ball shaped magical item to replace the sun and moon.