Shin's Arch-Demon Fireball Infernalist

Levelling is much like most caster, we shoot fire, things die, we level.

Endgame However is a different beast. Once in endgame we Start all maps in Demon form and kill while it ramps up to 70-100+ stacks, then we disable the form and instantly reapply to start the Demon Flame ramp up once more. We are a Infernal Demon! Spreading large amounts of seemingly infinite damage scaling spells that is only limited to how much life regen you can acquire.

With proper timing of the Flames we can also make sure we are dealing insane damage when facing boss mobs and hard to kill rares.

SO! Straight away I'm not beating around the bush, Gear is the MOST IMPORTANT part of this build. However, only in end-game. During levelling you can use any es armour you can get ya hands on and any combo of weapon as long it is can cast spells.

Now for end-game! The Demon form is currently extremely hard to maintain. This is because of the 0.5% Life degeneration per demon flame (demon flame increases by one every second). This is elevated by the "Life Regeneration per second" stat that can roll on all armours apart from gloves. This stat is going to be your main concern when looking at gear, after element resists and followed by flat mana.
So obtain yourself the Ghostwrithe armour, a belt, a helm and boots with life regen and elemental resists.

Important to note, do NOT get any life on your endgame gear. The reason for this is the % degeneration. A lower life pool means a lower % of damage. Which is also the reason we use Ghostwrithe. We need to have as little life as possible.

As for gloves, get a pair that has the highest "gain x life per enemy killed". This will add to your life sustain while killing.
Now ANOTHER catch with demon form is the weapon/offhand slots get removed in the demon form. So this means any ES or resistances you have on them will NOT carry over. So keep this in mind while gearing for elemental resists.

Lastly as mentioned earlier, we need as much mana as possible to scale our Archmage gem damage, which scales per 100 mana. This is one of our main damage boosters.

We will start with levelling:

Use a staff with Firebolt and as you level add Scattershot and Unleash. You can pair this with flame wall and summon raging spirits until you get Fireball, then everything changes.

Once you have access to Fireball, link it with Scattershot, Unleash and if you can use a jewelers orb, buy and link Wildshards. This is the point of the game were everything becomes a joke and we steam roll through the acts.

Once you have your Unleashed Wildshard Fireballs, drop Summon raging spirits (they lag you out with how many spawn with the fireball) and flame wall as you wont need them anymore.

Add Flammability to the mix when fighting boss mobs to make it easier.

Once you get access to Archmage use it!

Also grab the Infernal Doggo first ascendency and link to Fire Exposure for a living bebuffer.

This will get you to endgame with little issue.

So now ill go over each Skill individually in more detail.

Fireball:
Scattershot>Unleash>Wildshards>Controlled Destruction

Fireball is our Main skill and what we will be using from Act 2 normal to endgame maps. Scatter shot is straight forward so i wont go in to much details apart from it adding 2 extra projectiles. Now lets talk synergy. Unleash stacks up duplicate shots of the spell 2 times and can unleash the duplicate at 50% damage of the original, linked to Wildshards which add a 20% chance to cast 8 extra projectiles in a circle around you and the fact fireballs themselves explode in to a circle of firebolts, you start to see the absurdity of this combination. I'm talking screens full of fiery projectiles destroying everything. BUT only 20% of the time. This makes it a super fun when it happens. Adding more damage with Controlled Destruction when you can will just improve the output.

Ember Fusillade:
Fastforward>Considered Casting>Arcane Tempo

This ones straight forward. We use it as added damage vs tough rares and boss's in endgame. Using Fastforward and Arcane Tempo adds to its speed of use and the Considered Casting adds allot more damage with a slight speed reduction.

Flammability
Magnified Effect>Cursed Ground

This makes Flammability a aoe placeable, which has its downsides like being stationary, but i find its ability to remain in a area (like a chokepoint) i need it and harder to miss with far outweighs its cons. So we use Magnified to increase the aoe, then Cursed ground to turn it in to a placeable.

Archemage
Unlinked

This is the big boy that allows us to melt even with a tiny firebolt. It adds a % of extra damage as lighting damage per 100 of our total mana to all the spells we cast. This can get pretty crazy as we get better gear. The down side is the extra mana cost. Each spell costs and additional % of our total mana (5.25% of our mana at gem 15) which adds up.

Infernal Hound
Fire Exposure>Meatshield>Elemental Army>Last Grasp

This is our goodest boy. We pop Fire Exposure on him to get his auto ignites to apply exposure to any enemy in range. This allows all our fire damage to hit just that much harder. After that we add all gems we can to keep him alive as long as we can. So Meatshield, Elemental Army and Last Grasp are a no brainer.

Demon Form
Secondwind>Ingenuity
This is the reason your here reading this. The demon form. Gem links are simple and just allow us to have use of the form faster and allows us to use it as soon as we need to reset the demon flames.

While in demon form, every second that passes adds a demon flame stack to your character which gives benefits and downsides which grow as the stacks grow.

The benefits of being in demon form are insane! You get %faster cast speed per flame, %spell damage per flame and a flat increase to spell levels.

this can scale extremely well and makes us a monster.

However, The downsides are heavy. We get a 0.5% life degen per flame which NEEDS to be managed otherwise we will burn through our life faster then we can burn the enemies (see gear and tree).
Maintaining Mana also becomes allot harder as we cast faster and our skills are MUCH higher level with their higher level costs.

I use Mobalytics tree, so you can see it here: https://mobalytics.gg/poe-2/profile/b1ad00fb-3a93-47a1-ba03-cb1b4f8e95ce/builds/d3c38145-3d7e-4352-bf5b-af8b15eacec6

Here ill explain the levelling order of the tree and give options on when to take certain nodes. This will get you to level 75 at which point you can go nuts and get whatever. Id suggest more elemental nodes.

*Important
While levelling, do NOT pick up the item from Ogham Manor - Candlemass, The Living Right Boss. First is 20 flat life and crual version is 8% increase life. You mist use it if you pick it up and if you want to MIN-MAX its best you don't.

The below will be broken up in to 3 Sections
Main path
Post Archmage path
Demon Form path

Main path:

We start out by taking the Energy shield path on the right side of the witch start, grabbing Pure Energy, Dampening Shield and Mana Blessing as you head north.
Now make your way all the way to the top of the tree and take a left to the large Fire cluster. Here we take 2 branches, Volcanic skin and Cremation. THis will give us a very large boos to damage that will persist thought act 1 and 2.

Nest we want to work on out defence as we will need it about now. Moving back south through the main path we took, you will see a large Energy Shield cluster on the left. That's our goal. We want to grab Illuminated Crown and Heavy buffer (this adds to out lower life pool) to make us a bit tankier.

Nest we want to add to the tank grab Energy shield recharge circle to the top left of the witch start area. This will help us maintain out Energy shield as long as we don't keep getting hit.

Next to the the above is the Elemental ailment "arrow", Here you will take the left side and then grab Exploit the elements.

Now we are pretty solid but you may have noticed at this point, we are getting stunned allot. Time to fix that. From the witch starting location and Energy shield nodes, we take a right and head all the way to the stun arms and grab Asceticism and Self Mortification. Boom, no more stun issues.

Now its jewel time. We have 2 slots and we want 1 to add Ignite chance and the other shock chance. That's all. Any other mods will just be a boon.

Now we can grab the Echoing flames that's slightly to the south.

Now we have a choice. GO Demon Form path or Archmage path. Either is fine. But i went Archmage before demon.

Post Archmage Path:

Now we have Archmage online we want to buff it up. While we are near Echoing flame, Go ahead and grab Echoing thunder as well. That will allow us to do more damage when we shock guys, and thanks to our jewel, its often.

Near by is the Alternating Current node, we want this, grab this. It will increase the damage debuff shock applies and give us some much needed mana regen.

Next we head to the top right of our connected tree. We are aiming for Raw Mana and Arcane Intensity. This will boost our mana, mana regen and damage. Good Stuff all round.
Now we just need to remove pesky elemental resists. We do this by grabbing the nearby Electric Amplification then add more damage with Pure Power.

Demon Form Path:
Here our main goal is to get Life Regeneration Rate, this allows us to maintain Demon form longer, and a longer demon form means more damage.

So we want to head from the Exploit the elements "Arrow" and head south to grab the Full recovery node. From there we continue south and hang a right to grab Devoted Protector and Resilient soul. These will give us 35% faster regeneration rate. This may not sound like much, but it adds up.

Extra note:
Now that we have our tree, lets make sure we dont have any more str then we need. Devoted Protector gives us 10 STR so we may have STR we can get rid of. Do that. STR gives Life, we don't wont Life.

Acendancys:
First, Grab our best boy the Infernal Hound for the free -20% damage reduction and free Fire exposure when linked.

Second grab the Demon form. This is as far as we will go on this branch as the next caps the demon flame at 10 and we aint no 10 Flame casual!
Third and Forth are Altered flesh and Baidat hand. These are just straight up tank nodes and we already got damage covered.