Are you affected by this spell if you're within the area after you've succeeded on the saving throw? The wording is confusing.

No. In order to be illuminated the target(s) must fail the saving throw.

If a creature effected by the spell leaves the 20 foot area of the cast radius while still concentrating, is it still glowing and subject to the spell, or do you have to keep the creature in the area for it to continue to work? I'm unsure cause they use the phrase, "for the duration"  but not "that remain in the area of effect." I always kind of saw it as you toss glow paint on everything but have seen it interpreted both ways.

Thoughts?

I always played it as once the dex save failed, the target is illuminated regardless of where it goes thereafter, provided concentration is maintained.   After the spell is cast, there is no longer an area of effect per se, just those creatures affected by the spell for the duration wherever they may be.  

Quote from Todd_PC_DM >>

I always played it as once the dex save failed, the target is illuminated regardless of where it goes thereafter, provided concentration is maintained.   After the spell is cast, there is no longer an area of effect per se, just those creatures affected by the spell for the duration wherever they may be.  

 Imagine a "magic paint-ball" (you get to choose DIFERENT colors - go rainbow wyld!) once "marked" the creatures "stay painted" FOR THE DURATION of the spell... so... At least that is how I see it. BUT DM could "limit" that too, IF necesary (and the spell casters are abusing it) To an AREA of the initial "range/aoe", pinned to the center "when casted"... bu would give ALSO the benefit that IF some foe entered could fail teh save throw AND be marked (one for one, I guess). As it stand for me, at least, it's "magic paint ball" with a plus, the enemies "shed light" for 10 foot radius! "Snipey-snipeies and Stabby-stabbies" go ape-crazy (me included). Killed bunch of gobos this way.

What is the dc?

I always imagined it as the target getting covered in glitter if they fail the DC

Can this be upcast?

Quote from AlexBolton >>

What is the dc?

 It says in your spell casting tab, typically its 8 + proficiency bonus + spell casting ability.

Any creature in the area when the spell is cast has to make the saving throw or be lit up. Once lit, they are lit for the duration, as you said. Leaving the original area of the casting changes nothing, nor does entering the area of the casting after the spell is cast. The only way for a creature to end the affect early once affected is to break your concentration on the spell.

The DC depends on the caster. A 3rd level wizard casting darkness with an Intelligence of, say, 16 (+3) and a proficiency bonus of +2 will have a spell DC of 13 (8+Int+Prof, or 8+3+2).

That same wizard casting darkness at 20th level with an Intelligence of 20 (+5) and a proficiency bonus of +6 will have a DC of 19 (8+5+6).

How would this affect a invisible stalker?

The advantage from Faerie Fire and disadvantage from being invisible would balance out, and thus be straight rolls.

'and the affected creature or object can't benefit from being invisible.'

So they could not be invisible any longer, you would have advantage on attack rolls

Quote from B4tchieBoy >>

How would this affect a invisible stalker?

 Yes, in fact this spell is a hard counter to the Invisible Stalker.  It would be wise to prepare it if you think you are going to encounter one.

does this spell work in magical darkness? 

I would say no since it isn't as bright as say the daylight spell but more like a candle in terms of brightness. And simular to the light spell it still gets over powered by magical darkness.

ok thank you!!

 I'm a little confused. It does not specify each object not worn or carried, so are all objects on a  creature lit up even if the creature itself is not lit up?

Casting word is:

Glowicus Drablocus