I was trying to create my character with the dndbeyond character builder, but I can't figure out how to use it to create a demigod-type. Is the builder limited or can I simply not create such a being?

No, you can't play a demigod, so you can't build one in the character creator.

Any PC is practically a demigod from 17th-20th levels.

"17th-20rh levels"?

Does that mean I can only build my way up there or can demigod-status be the basis of the character? 

The whole point of an RPG is for your character to start out at the bottom and gradually work their way up in power.  Starting out as a character who's already super powerful and just being able to trample everything in your path would get boring quickly.

I see what you mean, but what if it was merely a character description, and did not necessarily mean it was ridiculously powerful?

Quote from OdrowaZ90 >>

I see what you mean, but what if it was merely a character description, and did not necessarily mean it was ridiculously powerful?

 
Then make any character you like. What is it specifically you are going for? Hercules? Achilles? What?

I feel protective of my concept, but best put it down here so it's on record. I'm thinking fallen deity, sentenced to wonder the mortal realms (as an immortal), working towards redemption. Therefore the character is already ancient. Divine but highly limited. I shouldn't make it super powerful, though I wish there was a factor that gave it a LITTLE extra edge from the start.

That’s hardly a unique concept, and it really doesn’t fit D&D well. The rule of thumb is that the most interesting things that happen to your character shouldn’t be in their backstory. But, if I wanted to try to make something like that I would probably go with a Divine Soul Sorcerer.

Well what if I didn't care if such character ever found redemption during the game, but it was what drove said character to fight for good? And I left the redemption thingg up to the DM, not caring one way or the other.

A background can help with this.  Haunted One would be relatively fittings - townsfolk see something horrific about you and feel compelled to help you, as they might due to a divine presence (even one that has lost the bulk of its divine mantle).  Additionally it gives you arcana and religion proficiencies, which would be rather fitting. If your DM allows custom backgrounds, you could take the Haunted One feature and utilize religion and history as your skills, which would likewise be appropriate. 

Alright, but the character builder on this site simply doesn't have "demigod" in its system? Or is that even the right term for a fallen divinity (as opposed to a ½God ½human being)?

Quote from 6thLyranGuard >>

The whole point of an RPG is for your character to start out at the bottom and gradually work their way up in power.  Starting out as a character who's already super powerful and just being able to trample everything in your path would get boring quickly.

 
I whole heartedly disagree.

I've played several games where we've started at 17th level or higher. A few starting at 20th. And one starting 20th with 3 epic boons each, loaded with magic items and unlimited money.

In all it has been an incredibly fun experience, engaging and challenging. Even the one with multiple legendary items and epic boons was very challenging. We neared got TPK'd at least 3 times and one of those times required us to retreat and try an entirely different strategy. 

The fun of a game is decided by you and the DM, not by your level. Higher level requires more skill by the DM, yes, but any 20+ level campaign is very doable and incredibly entertaining and can be very difficult. 

Ever read superhero comics like Marvel or DC? Even characters, actual demigods, actual gods, and beings able to rewrite reality itself - still get challenged, still struggle, and still presents interesting stories. It's imagination. There is no limit. It's the same with D&D. A party of Level 100 characters compared to the DM would be like comparing a child's sandbox to the Sahara. No matter how powerful your character may be, the DM can provide enemies that are more so. No matter how skilled your character is or what resources they have, the DM can create taxing and difficult scenarios.

Character level is ultimately irrelevant to what fantastic and fun experiences can be had with D&D.

So it is less, "20th level is boring" and more that your DM doesn't have the skill to handle 20th level stories.

It would not really be the right term, but D&D has a.... let's say loose relationship with using terms correctly (here's looking at you Longsword).  

If you make a custom background, you can name it "Fallen God" or "Demigod" or the like; the name is a flavor element that does not change anything in game--it's the choices on background that matter.  

Quote from OdrowaZ90 >>

Alright, but the character builder on this site simply doesn't have "demigod" in its system? Or is that even the right term for a fallen divinity (as opposed to a ½God ½human being)?

 
Why would it? D&D 5th Edition doesn't have "demigod", so why would this site?

You can try Aasimar race (humanoids descended from Celestial beings).
There's "Divine Soul" Sorcerer subclass for that 'I have divine power in my blood/soul' or 'I am the chosen one!' type.
There's 'The Celestial' Warlock patron subclass for somebody who has gained divine power by arcane means - a gift of a celestial, perhaps.
There's the Cleric class to wield divine magic as a servitor of Gods.

And more.

And any of the above can be flavoured as you need - demigod, avatar of your true God self, reincarnation of a past defeated God striving to return to former power and status, and so on.

There is no "I click Demigod", no. But try some imagination, perhaps, and reflavour something to fit. 

Quote from Cyb3rM1nd >>

Quote from 6thLyranGuard >>

The whole point of an RPG is for your character to start out at the bottom and gradually work their way up in power.  Starting out as a character who's already super powerful and just being able to trample everything in your path would get boring quickly.

 
I whole heartedly disagree.

I've played several games where we've started at 17th level or higher. A few starting at 20th. And one starting 20th with 3 epic boons each, loaded with magic items and unlimited money.

In all it has been an incredibly fun experience, engaging and challenging. Even the one with multiple legendary items and epic boons was very challenging. We neared got TPK'd at least 3 times and one of those times required us to retreat and try an entirely different strategy. 

The fun of a game is decided by you and the DM, not by your level. Higher level requires more skill by the DM, yes, but any 20+ level campaign is very doable and incredibly entertaining and can be very difficult. 

Ever read superhero comics like Marvel or DC? Even characters, actual demigods, actual gods, and beings able to rewrite reality itself - still get challenged, still struggle, and still presents interesting stories. It's imagination. There is no limit. It's the same with D&D. A party of Level 100 characters compared to the DM would be like comparing a child's sandbox to the Sahara. No matter how powerful your character may be, the DM can provide enemies that are more so. No matter how skilled your character is or what resources they have, the DM can create taxing and difficult scenarios.

Character level is ultimately irrelevant to what fantastic and fun experiences can be had with D&D.

So it is less, "20th level is boring" and more that your DM doesn't have the skill to handle 20th level stories.

 It's not that 20th level is boring.  It's that starting at 20th level and having your character never grow or change in any meaningful way for an entire campaign is boring.

Yeah, a Fallen Aasimar Divine Soul Sorcerer with a custom background would be my build for this concept.

Quote from 6thLyranGuard >>

Quote from Cyb3rM1nd >>

Quote from 6thLyranGuard >>

The whole point of an RPG is for your character to start out at the bottom and gradually work their way up in power.  Starting out as a character who's already super powerful and just being able to trample everything in your path would get boring quickly.

 
I whole heartedly disagree.

I've played several games where we've started at 17th level or higher. A few starting at 20th. And one starting 20th with 3 epic boons each, loaded with magic items and unlimited money.

In all it has been an incredibly fun experience, engaging and challenging. Even the one with multiple legendary items and epic boons was very challenging. We neared got TPK'd at least 3 times and one of those times required us to retreat and try an entirely different strategy. 

The fun of a game is decided by you and the DM, not by your level. Higher level requires more skill by the DM, yes, but any 20+ level campaign is very doable and incredibly entertaining and can be very difficult. 

Ever read superhero comics like Marvel or DC? Even characters, actual demigods, actual gods, and beings able to rewrite reality itself - still get challenged, still struggle, and still presents interesting stories. It's imagination. There is no limit. It's the same with D&D. A party of Level 100 characters compared to the DM would be like comparing a child's sandbox to the Sahara. No matter how powerful your character may be, the DM can provide enemies that are more so. No matter how skilled your character is or what resources they have, the DM can create taxing and difficult scenarios.

Character level is ultimately irrelevant to what fantastic and fun experiences can be had with D&D.

So it is less, "20th level is boring" and more that your DM doesn't have the skill to handle 20th level stories.

 It's not that 20th level is boring.  It's that starting at 20th level and having your character never grow or change in any meaningful way for an entire campaign is boring.

 
Again, disagree.

Levels only progress a character as a bunch of stats (and you can continue to gain these, past 20th level) and powers (and you can continue to gain these past 20th level, see Epic Boons).

But defining the progress of a character by some numbers is limited. Progressing a character through story, narrative, personality? Possible at any level. You still have flaws, still have goals, still have challenges external and internal to overcome. Again, the comics feature many characters that a few THOUSAND times more powerful than a 20th level D&D char. And yet, for many years, we read their adventures going through their ups and down, the hurdles they face, their failures and their accomplishments. You can do the same in D&D. Just think less about the character sheet and "what my character can do" and think more, instead, about "who my character is" - and just like that, level becomes an entirely irrelevant factor in your character's story.

Quote from Cyb3rM1nd >>

But defining the progress of a character by some numbers is limited. Progressing a character through story, narrative, personality? Possible at any level. You still have flaws, still have goals, still have challenges external and internal to overcome. Again, the comics feature many characters that a few THOUSAND times more powerful than a 20th level D&D char. And yet, for many years, we read their adventures going through their ups and down, the hurdles they face, their failures and their accomplishments. You can do the same in D&D. Just think less about the character sheet and "what my character can do" and think more, instead, about "who my character is" - and just like that, level becomes an entirely irrelevant factor in your character's story.

 Comic books are a fundamentally different medium than tabletop roleplaying games.  What works in one medium doesn't mean that it will necessarily work in another, that's why a particularly faithful adaptation of a novel can wind up being a bad movie.

And even in comic books, the heroes tend to start out young and inexperienced, unsure of themselves and unskilled with their powers.  Even characters like Superman get the origin story where he's not too good at flying, he isn't a skilled fighter, and he's not at his physical peak.  They've got to grow too: the audience tends to complain when a character shows up out of the blue and starts easily thrashing top-tier villains like Darkseid or Ultron instead of having them start out initially facing off against lower-tier bad guys who they struggle to beat.

Just to be clear to the OP since they said they are new (and since the thread is getting derailed by a tangential and likely fruitless debate):  There is no right or wrong way to play D&D, so long as you and your group are all having fun.  You want to have a character who was once something grand, and now isn't?  That's perfectly fine--and you can easily make your story of learning to live among mortals interesting, compelling, and fun to play.  

That's the magic of D&D--you are given a blank canvas to write whatever story you want.