The fields ablaze, the lone woman watched the temple burn down, the flames licking the walls. As a child, she was fascinated with fire and, despite how cruelly she was branded as punishment, the flame's touch only made her stronger. She knew the perfect accelerant, the right spot to start the flames, and the right material to burn down in an hour that took teams of men years to build. 

The Arsonist is an archetype of rogue for those with a belief in the power of fire that is so unnatural, so intense, it manifests far more destruction than what a simple flame should do

Spark’s Knowledge

Starting at 3rd level, your connection to fire manifests in an innate talent for alchemy and fire-starting. You gain proficiency in alchemist's supplies and are able to produce the effects of a Tinderbox with only two pieces of flammable material that are at least two inches in length as an action. 

At 9th level, you gain expertise in alchemist's supplies and the gold and time you must spend to make one dose of alchemist's fire is halved.

Searing Strikes

When you choose this archetype at 3rd level, you learn how to weave burning flames into your strikes. When you make an attack roll with a weapon you are proficient in, you may choose to deal fire damage instead of the attack's normal damage types.

You also gain an additional way to use your Sneak Attack; you don't need advantage on the attack roll to use your Sneak Attack against a creature if the creature has taken fire damage within the last round of combat and you don't have disadvantage on the attack roll. All the other rules for Sneak Attack still apply for you.

When you reach 13th level, when a creature is reduced to 0 hitpoints from fire damage dealt by your Searing Strikes, their body is reduced to ash.

Scorched Earth

At 9th level, you have mastered the knowledge that a fire is only as powerful as its fuel. Over the course a long rest, using your alchemist's supplies, you can produce a number of flasks of fire accelerant equal to your proficiency modifier, provided you have enough flasks. 

A creature may spend one minute and use one flask of accelerant to cover up to a 30 foot radius circle of land in the flammable fluid. If an effect that deals fire damage enters the area covered, it immediately goes up in flames and replicates the effects of the Create Bonfire cantrip for a duration of 1 minute. The saving throw DC for the effect equals 8 +proficiency modifier+your Wisdom modifier. Any non magical material in the flames is destroyed after one minute.

Additionally, as an action a creature may use one flask of accelerant to fuel a torch with otherworldly brightness. A torch affected by the accelerant can suppress magical darkness within 20 feet of it and can be lit and burn underwater. When lit in this manner, it lasts for one hour.

Blaze and Smoke

You become so intertwined with the fire you create that you learn to resist it's effects and even use it to protect yourself.  Beginning at 13th level, You gain resistance to fire damage, and are immune to the effects of extreme heat (as described in the DMG).

Additionally, when you use your Uncanny Dodge from an attacker within 30 feet of you that you can see, you can throw a smoke screen as part of that reaction at the creature that made the attack. The target creature must make a successful Constitution saving throw or be blinded until the start of their next turn. The DC for this saving throw is equal to 8 + proficiency bonus+ your wisdom modifier.

You can use this feature a number of times equal to your proficiency bonus, and you regain all expended uses when you finish a long rest.

Roaring Inferno

At 17th level, your works of arson have reached the stuff of legend, and such, you have succeeded in learning how to burn the unburnable. Your Searing Strikes treat immunity to fire damage as resistance and resistance to fire damage as normal, and when attacking a creature that is neither immune or resistant to fire damage with your Searing Strikes, the target suffers half of the damage from the attack at the end of its next turn.