Path of Dispater

You fight in the name of Dispater, the Iron Duke.  Dispater is the lord of the Iron City of Dis on the second layer of Hell, also called Dis.  His home is the Iron Tower, an impregnable fortress made to protect a paranoid archdevil.  Dispater is the picture of caution and careful thought.  He never acts rashly, never leaves his tower (unless commanded to do so by Asmodeus), and always has at least nine different backup plans and escape routes at any given moment.

Your "rage" is not a surge of anger, but represents Dispater giving your flesh the hardness of iron.

Ironmonger

Like your paranoid patron, you prize defense above all.  At 3rd level, you gain proficiency in heavy armor.  While wearing heavy armor, you still gain the benefits of Rage and Fast Movement.

Iron Mind

Beginning at 6th level, you can't be charmed or frightened while raging. If you are charmed or frightened when you enter your rage, the effect is suspended for the duration of the rage.

Strength of Iron

Starting at 10th level, Dispater teaches you the ultimate defense.  If you fail a saving throw, you can choose to succeed.  Once you use this feature, you can’t use it again until you finish a long rest.

Conjure Iron Tower

Beginning at 14th level, you can use an action to summon a sturdy iron tower.

An iron tower erupts from a square area of ground of your choice that you can see within range. There must not be any buildings or other structures on it. You have great flexibility in choosing the tower’s design.  The tower has a square base of 10-20 feet on a side and is 20-60 feet tall. Any creatures in the area are harmlessly lifted up as the tower rises.

The tower could be completely empty inside, or it could have up to twenty floors with 3-foot-high ceilings that force creatures to crawl.  Different floors can have ceilings of different heights.  Each of the floors can be divided into as many rooms as you like, provided each room is at least 5 feet on each side. The floors of the tower are connected by iron staircases, its walls are 6 inches thick, and interior rooms can have iron doors or open archways as you choose.

The tower features numerous arrow slits, machicolations, and torture devices. If you desire, it can have a hidden basement (DC 22 Investigation check to find it) 10 feet below the ground floor. The tower may have up to four sets of thick iron doors that open either inward or outward as you please. 

The tower has spartan furnishings, and it contains sufficient food to serve for up to 20 people each day. Furnishings, food, and other objects created by this spell crumble to dust if removed from the tower.

A staff of a dozen damned souls obeys any command given to them by creatures you designate when you cast the spell. Each soul functions as if created by the unseen servant spell.  

The tower is made of iron that can be damaged. Each 10—foot—by a 10-foot section of iron has AC 15 and 40 hit points per inch of thickness. It is immune to poison and psychic damage. Reducing a section of iron to 0 hit points destroys it and might cause connected sections to buckle and collapse at the DM’s discretion.

The tower radiates a strong evil aura and each side prominently features the symbol of Dispater.

After 7 days or when you use this feature somewhere else, the tower harmlessly crumbles and sinks back into the ground, leaving any creatures that were inside it safely on the ground.
Using this feature on the same spot once every 7 days for a year makes the tower permanent.

Once you use this feature, you can’t use it again until you finish a long rest.

You are encouraged to replace the 20th level feature Primal Champion with the following ability:

Talos Swap

At 20th level, you have proven such a steadfast champion of Dispater that the archdevil sees you as an object worthy of protection.  However, such is his paranoia that he would never reduce his own defenses for your sake.

As an action, you can teleport to Dispater’s Iron Tower in the City of Dis.  You remain there for 1 minute, guarding.  The Tower rarely falls under attack, but what happens while you are there is up to the DM's discretion.

In your place, Talos the Triple Iron Golem is sent.  He appears on a point of your choice within 50 feet of where you were before going to Hell.  He will fight to crush your foes as long as you remain in Hell.  Talos' statistics can be found on DndBeyond.

At the end of 1 minute, Talos returns to protecting Dispater and you reappear in the same square that you originally were in, or if it’s occupied, the nearest unoccupied square