I have a simple question about this spell, nothing related to damage or saves or applications. But it says "The flames don't harm you." Does this apply also to the possessions you have on your like your clothes, potions, money, weapons, etc?

Yes. Typically, things that are worn or carried aren't affected by spells, effects, attacks, etc.

Awesome, thanks. I assume that applies to most spells then, unless specifically stated.

Can you just move by each enemy on your turn to make them all take damage? 

All red dragons in my world can and will cast this spell for hype.

Due to the wording "that moves within 5 feet of you" its a matter of debate. Some DMs would be fine with it, others would say that they have to move within 5 feet of you themselves. It doesn't hurt to bring it up though.

I wish the description clarified whether the line of fire stays for a while or disappears instantly...

Quote from RestlessJon >>

I wish the description clarified whether the line of fire stays for a while or disappears instantly...

The answer is in the omission. No, it does not stay, otherwise it would say so.

Basically, this spell lets you say FLAME ON!

the investiture spells are pretty cool wish they were a little less underated

I have a question about this spell and I would appreciate some wisdom from you intelligent sages.  Let's say you activate this spell and then go stand next to an enemy.  On the enemy's turn, they move away from you.  Would you interpret that has them having moved "within 5 feet of you for the first time"?  I interpret it that they did indeed move, and they moved while they were within five feet of you.  If they didn't move away from you, but instead decided to move around you so they are standing behind you, they would still take the damage since they moved while they were within 5 feet of you, but they wouldn't take the damage multiple times since it says "for the first time."  Am I right?  Or do they only take the damage if the enemy chooses to move into your space on their turn?

They have moved while within 5ft of you, but they haven't moved to be within 5ft, so it depends on your GM's interpretation. The Moon Druid in my group was OK with the more restrictive version, because it's still a good aura, especially to get people to avoid going near you on low HP.

The real value of this is of course IMMUNITY TO FIRE. My Abjurer, who is never closer than a misty step from the enemy, had this up before fighting an Ancient Red Dragon. It was awesome.

Quote from Yselieg >>

They have moved while within 5ft of you, but they haven't moved to be within 5ft, so it depends on your GM's interpretation.

A creature is only considered to have moved if it uses its own movement to do-so; so unless you magically compel them to move it won't work, neither will shoving or otherwise pushing enemies, in the same way that those don't trigger attacks of opportunity.

You need to either encourage enemies to come to you (i.e- by making it clear they'll take more damage if they don't) or go to them and hope they don't just move away, though you can of course fire beam them each time as well for good measure.

Here's how the passive damage effect of this spell works:

- On your turn, with the spell active on you, you can run around anywhere you want.  No matter how close you get to any other creature, no damage is done.

- On the beginning of the creature's next turn, if it is within 5' of you then... nothing happens yet.

- The creature has the option to move away (or not away), in which case it triggers the "moving within 5 feet" rule and immediately takes damage.  This only happens once, even if the creature moves away, comes back, takes an action and then moves again.  The damage is only done the first time makes a move which starts in, ends in, or passes through the area of effect.

- If the creature did not move and ends its turn still within five feet of the caster, then it will take damage at the end of the turn, after it has completed any other actions it was taking.  This does not happen if it already took damage from moving, and there is no way that a creature can be damaged twice in the same turn by the same effect.

- Because damage is only done when a creature "moves within 5 feet of you for the first time on a turn or ends its turn there", a creature could avoid damage entirely by using a spell such as Misty Step, Dimension Door or Blink to relocate itself outside of the area of effect without taking a "move" action.  Since it never moved and did not end its turn within five feet of the caster, the damage is never triggered.

Note that the same rules would not apply to spells like Wall of Fire (which only "deals fire damage to each creature that ends its turn within 10 feet of [the hot side] or inside the wall", and doesn't care about movement taken outside of the wall itself.) or Spirit Guardians (which does damage "when the creature enters the area for the first time on a turn or starts its turn there").  Each spell acts a little differently, so be sure to read the description carefully.

I’m going to post this question here because both the wall of force and the fireball comment sections were too filled up but here it is

if I were to make a dome of wall of force around two people one of which being a sorcerer immune to fire damage who would then cast a 9th level fireball what would happen to the fire balls damage as it has a 20 ft blast but the dome is contained to 10 ft and nothing can pat through it so what would happen to the damage

RAW, nothing exciting. The Fireball would be contained within the dome, but still do its normal damage, because there's no rule about compressed spaces in the spell description.

So if I cast this concentration spell, and then wild shape into whatever, is the verbal component in the casting of the spell, or do I still need to be able to speak to create the line of flame?

Nope, you don't need to be able to speak to create the line of flame RAW. You cast the spell once at the start with the verbal and somatic components as normal, it then gives you access to a new action that creates the line of fire. Note the line of flame would still be a magical effect, since it's created by the effect of a spell, but taking the action to create the flames is not itself a spell you cast.

Quote from KaptainKittens >>

Nope, you don't need to be able to speak to create the line of flame RAW. You cast the spell once at the start with the verbal and somatic components as normal, it then gives you access to a new action that creates the line of fire. Note the line of flame would still be a magical effect, since it's created by the effect of a spell, but taking the action to create the flames is not itself a spell you cast.

 I rolled the dice last game before you answered :-D  After grappling, my giant scorpion basically cooked its own meal that it was holding.  Thanks for the answer though.

If we ever get back on the water, I want to use it to have a flaming whale breach and capsize a boat

.... so this spell is Concentration. I can get the same defensive power with Fire Shield (no Concentration and 4th level) and more burn damage. And I can save my 6th level slot for Disintegrate? Zoinks.