There's more Fist of Rhalgr surviving, they just appear to be in hiding (I suspect them to come out and riot next patch though. Crossing my fingers). They continue to state that during the questlines several times over and over.
So I'd... not go the route of going "Well, my character was trained by the last surviving nobility of Ala Mhigo", because eh, while no one will stop you, I think some random, made up older Monk NPC might be a lot less... controversial.
Or you could just avoid the problem all together, and for an older Fist who simply was at one of the 4843743874873 temples they had. Besides, I personally find older Fist a lot more believable due to the fact they had to endure an absurd training from childhood on. Your character will simply have lacked the community, temples, scriptures ect ect. So they are unlikely to be on par with the Monks who were trained from childhood to adulthood back in Gyr Abania's temple. (Plus you get to roleplay all that delicious drama and conflict regarding the purge of your Brother and Sisters, the resentment, guilt, anger. WHY YOU WANT TO MISS OUT ON THE BEST PART I STILL DON'T KNOW? Older Ala Mhigans are so much more interesting to me in general. Because all the super interesting lore and events happened around the King of Ruin. Anyhow...)
But each to their own! I'd still suggest not going with the quest NPC, due to your character well... being close to a pretty important NPC. Just make sure that you read up about Monks, as they are still very much indeed simply Rhalgr's priests. Who can kick you in the face. No really, there's quite a few 'guidelines' with Monks, in terms of what you cannot and can wear, whether you can have a soulstone ect yadayada. Heresy still be a real thing for them after all!
So I'd... not go the route of going "Well, my character was trained by the last surviving nobility of Ala Mhigo", because eh, while no one will stop you, I think some random, made up older Monk NPC might be a lot less... controversial.
Or you could just avoid the problem all together, and for an older Fist who simply was at one of the 4843743874873 temples they had. Besides, I personally find older Fist a lot more believable due to the fact they had to endure an absurd training from childhood on. Your character will simply have lacked the community, temples, scriptures ect ect. So they are unlikely to be on par with the Monks who were trained from childhood to adulthood back in Gyr Abania's temple. (Plus you get to roleplay all that delicious drama and conflict regarding the purge of your Brother and Sisters, the resentment, guilt, anger. WHY YOU WANT TO MISS OUT ON THE BEST PART I STILL DON'T KNOW? Older Ala Mhigans are so much more interesting to me in general. Because all the super interesting lore and events happened around the King of Ruin. Anyhow...)
But each to their own! I'd still suggest not going with the quest NPC, due to your character well... being close to a pretty important NPC. Just make sure that you read up about Monks, as they are still very much indeed simply Rhalgr's priests. Who can kick you in the face. No really, there's quite a few 'guidelines' with Monks, in terms of what you cannot and can wear, whether you can have a soulstone ect yadayada. Heresy still be a real thing for them after all!
