The Ashen Wolf

You've made a pact with a primal spirit of fire and the hunt; a scourge of the forests and fields, and a harbinger of ill fate. This creature is wise and cunning, making deals with those who have the potential to bring ruin and change. Some consider the beast to be a native to the Plane of Elemental Fire; others call it a fiend from the darkest hells. Whatever the origin, the ember-eyed wolf often extracts payment in the form of wishes gone awry: a demand for power becomes a battle to retain one's humanity, while a request for a peaceful resolution often has the price that brings constant regret. Expanded Spell List
The Ashen Wolf lets you choose from an expanded list of
spells when you learn a warlock spell. The following spells
are added to the warlock spell list for you.
Ashen Wolf Expanded Spells 

Spell Level Spells
1st inflict wounds, longstrider, burning hands*
2nd flame blade, pass without trace
3rd Melf's minute meteors, haste
4th fire shield, wall of fire
5th Hallow*, Flame Strike*

* not originally part of subclass 

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Breath of Smoke
Starting at 1st level, you can exhale a 15-foot long cone of blistering embers as an action. Creatures within the cone must make a Constitution saving throw against your warlock
spell save DC. Creatures that fail take fire damage equal to your warlock level + your Charisma modifier, and are blinded for one minute. They can repeat their saving throw at the end of each of their turns, ending the effect on a success. Once you activate this feature, you cannot do so again until you finish a short or long rest. Additionally, you can breathe and see normally in smoke or ash-filled air.

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Feast for the Fire
Beginning at 6th level, the Ashen Wolf empowers you with a fragment of its terrible hunger. Whenever you move during combat, you can choose to leave behind a trail of ash that
hangs gently in the air, lightly obscuring the area until the start of your next turn. Whenever a hostile creature enters this trail, you can use a reaction to summon a set of flaming fangs to bite them. Make a melee spell attack roll against the target. If you hit, the target takes 1d10 fire damage.

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Endless Pursuit
At 10th level, your patron grants you a portion of its primal stamina. You gain resistance to fire damage. Whenever you finish a short rest, you can choose to reduce your level of exhaustion by one. You can do this twice, and you recover these uses when you finish a long rest.

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 The Hounds of Hell
At 14th level, you unlock the power to incarnate in the form of your patron. As an action, you assume the form of a Medium or Large canine creature wrapped in flame. Your statistics are unchanged, and your equipment merges into your new form but still functions as normal. For 1 minute, or until you exit this form as a bonus action, you gain the following benefits.

• You ignore your current level of exhaustion.
• Your speed increases by 20 feet.
• You have advantage on melee attack rolls against a creature if at least one of your allies is within 5 feet of the creature and the ally isn't incapacitated
• You are immune to fire damage.

When you use this feature, two hell hounds (MM pg. 182) burst forth from your burning flesh into spaces within 5 feet of you. They respond eagerly to your commands for the duration of the change, and are elementals, not fiends. When you exit this form, you immediately attempt to reabsorb the hell hounds. The hell hounds instantly fall apart as ash and swirl back to you from up to 300 feet away. If one hell hound has died or cannot return due to distance, you gain one level of exhaustion. If both are absent, you gain two levels of exhaustion and are stunned for 1 round.
Once you use this feature, you cannot do so again until you finish a long rest.

 

All credit  goes to (/u/GenuineBelieverer) and their Compendium of Forgotten Secrets

Expanded Spell List

The Fiend lets you choose from an expanded list of spells when you learn a warlock spell. The following spells are added to the warlock spell list for you.

Fiend Expanded Spells

Spell Level	Spells
1st	burning hands, inflict wounds, longstrider
2nd	flame blade, Pass without Trace
3rd	melf's minute meteors, haste
4th	fire shield, wall of fire
5th	flame strike, hallow

Breath of Smoke

Starting at 1st level, you can exhale a 15-foot long cone of
blistering embers as an action. Creatures within the cone
must make a Constitution saving throw against your warlock spell save DC. Creatures that fail take fire damage equal to your warlock level + your Charisma modifier, and are blinded for one minute. They can repeat their saving throw at the end of each of their turns, ending the effect on a success. Once you activate this feature, you cannot do so again until you finish a short or long rest. Additionally, you can breathe and see normally in smoke or ash-filled air.

Feast for the Fire

Beginning at 6th level, the Ashen Wolf empowers you with a fragment of its terrible hunger. Whenever you move during combat, you can choose to leave behind a trail of ash that hangs gently in the air, lightly obscuring the area until the start of your next turn. Whenever a hostile creature enters this trail, you can use a reaction to summon a set of flaming fangs to bite them. Make a melee spell attack roll against the target. If you hit, the target takes 1d10 fire damage.

Endless Pursuit

At 10th level, your patron grants you a portion of its primal stamina. You gain resistance to fire damage. Whenever you finish a short rest, you can choose to reduce your level of exhaustion by one. You can do this twice, and you recover these uses when you finish a long rest.

Hurl Through Hell

At 14th level, you unlock the power to incarnate in the form of your patron. As an action, you assume the form of a Medium or Large canine creature wrapped in flame. Your statistics
are unchanged, and your equipment merges into your new form but still functions as normal. For 1 minute, or until you exit this form as a bonus action, you gain the following benefits.

• You ignore your current level of exhaustion
• Your speed increases by 20 feet.
• You have advantage on melee attack rolls against a creature if at least one of your allies is within 5 feet of the creature and the ally isn't incapacitated.
• You are immune to fire damage.

When you use this feature, two hell hounds (MM pg. 182) burst forth from your burning flesh into spaces within 5 feet of you. They respond eagerly to your commands for the
duration of the change,  When you exit this form, you immediately attempt to reabsorb the hell hounds. The hell hounds instantly fall apart as ash and swirl back to you from up to 300 feet away. If one hell hound has died or cannot return due to distance, you gain one level of exhaustion. If both are absent, you gain two levels of exhaustion and are stunned for 1 round.
Once you use this feature, you cannot do so again until you finish a long rest.