My first D&D character has been conceptualized! Nasus Neverlight: A 289 year old Loxodon Cleric of the Peace Domain, orphaned during an attack on their traveling party utilizing the perilous underdark. Found and raised by the Myconids, deity Psilofyr. Though not having darkvision could be an issue Nasus utilizes keen smell to navigate the underworld and help spirit others through the underdark that need the aid to save others from his own families fate. 

This is my first attempt at a character and am excited to start my first campaign in the coming weeks. What here doesn't work with lore/info. I did my research to some degree and none of this seems fully impossible. Though being a Loxodon that can live up to 450 years old where as the Myconids lifecycle is 24 years seems like it could be rough on a dude. It would make communing with the Myconids deity an necessity, since he is the only constant being in that community. 

I had a quick read through the wiki for Psilofyr (link here: https://forgottenrealms.fandom.com/wiki/Psilofyr) and it seems Psilofyr's main philosophy is one of renewal and his main goal is the continuation of the Myconid race so there is liekly to be an 'in' for you to explore.

As Psilofyr's cleric, you have the lifespan needed to oversee multiple generations of Myconids and indeed you may actually be a new form of Myconid yourself (DM permitting), maybe you actually died in the ambush you mentioned and Myconid spores animated your body or brought you back to life.

A Nature domain Cleric or Circle of Spores Druid might be easier for roleplay and spells but given Psilofyr is a mostly peaceful entity so the Peace domain should work ok. I might suggest having your spellcasting having a cosmetic effect where it appears you are maipulating Myconid spores to accomplish the spell effects but it depends on how comfy your and your DM would be with it.

Edit: you also have the possibilities of being a splinter personality of Psilofyr, he is renowned for splitting of sections of himself to speak with other people and entities so maybe you are just a form he took and forgot to absorb back into himself...he has several 'rogue' identities roaming around the Planes, but again this is largely DM permitting.

Sounds like an interesting concept, though a few possible issues.

The main one is the age. The backstory of a low level character is supposed to be a starting point for your story, not a story in itself. Yes, it does give a cool backstory of being a long term, multigenerational protector of your adopted people, but what character level is the game starting at? If it's low, like first or third as many new games begin at, nearly three centuries of serving as a protector against the dangers of the underdark should result in a much higher character level. You can get a little bit of stretch by saying that your tribe/village/etc is conveniently isolated in an area without as many threats as usual, but even then I would strongly recommend dialing the age down quite a bit. If you're starting at first level your character has mechanically zero experience which means they haven't fought anything of consequence or faced any significant adversity of other types that would pose a threat so you explicitly haven't protected or guided anybody. You can get to third level with just a handful of moderately challenging encounters so even with a low-danger environment you're looking at maybe a few decades of activity. If you're starting at tenth level or something then you can go ahead and disregard this, but otherwise you're character explicitly has very little if any experience so I'd dial that back to under a hundred years old. 

The other is much more minor, but should require a bit more thought at least. You simply can't navigate completely dark environments by smell. That can tell you there's something in the area, be it food or a predator or whatever, but if won't tell you exactly where it is at any given instant and it won't keep you from tripping over rocks or banging into walls. SA simple response to this is that a first level Cleric can cast the light cantrip as often as they need to. You just need to be careful about doing so in a way that might attract unwanted attention in the dark, which can easily mesh with being a wisdom based character who advises their people on when various courses of action are prudent or not; in fact it can make for an excellent analogy in some such cases.

On the subject of navigating in the dark, have a chat with your DM and maybe look into having bioluminescent fungi that fuction as torches, possibly a baby Myconid growing from a Staff that you can use or maybe see if you can flavour a Light cantrip to be a cloud of spores give everything a strange glow as you stroll around or that applifies the glow from the fungus you have growing form the staff. If you do a web search for 'bioluminescent fungi' you can get plenty of images to give a good idea of how to descibe it to other players when/if you use it.

If you don't have darkvision then light should logically be at the top of your list for cantrips unless you have some very convincing character reasons not to have it. Yes, it can be flavored numerous ways but the main idea it that it makes something glow in place of carrying around bundles of torches in dark environments. And if you live in the underdark everything is a dark environment. 

Also, to the OP: One other important part of a backstory for a new character is why are you adventuring? If your character has a responsibility to their home and people, what is motivating you to leave them to do whatever else you're going to be doing as part of the actual game? It could be as simple as the main plot of the campaign concerns something that poses a threat to your home and you're going out to stop it. Are you feeling in a bit of a rut after decades of the same old same old and scratching the itch of some wanderlust? Do you seek knowledge, wealth, or anything else of benefit to bring back and improve the lives of your people? Have you suffered some sort of disgrace and are adventuring to clear your name so that you can return to your people with your honor restored? This doesn't necessarily have to be super in depth, but at least enough of something you can refer to if somebody asks you why you've left your home to brave the dangers of adventure.

All in all, this is a pretty good character concept. You might want to think about lighting, because like others have mentioned, not having darkvision would be pretty debilitating if you live constantly in the dark. You might want to think about his life before the Underdark, but it really depends on what kind of story you want to tell.