Quote Originally Posted by The_Last_Dragoon View Post
It is not required to do optional dungeons such as Satasha (Hard), though if these dungeons were to get an extreme mode in a later update, then the previous versions of that dungeon would have to be done in order for the extreme version to unlock.
It is a good idea to do them anyway to unlock duty roulettes so you can get the daily soldiery and poetics bonus. You should also do all the trials(trials include hildibrand trials as well) to unlock the trials roulette. Also doing them each once will give you your own bonus sol for first time completion which will make it a lot easier to gear up as well - along with any drops you get out of them.

As for a relic reborn do the first 2 steps - get the relic(you get a title for this as well) and upgrade to zenith. It's a very easy way to get an ilvl90 weapon that can serve you while you keep gearing up and then obtain a better weapon.