This is a first-draft attempt at using the rules for infernal war machines in Descent Into Avernus to make a mechsuit for use in a 5e D&D Dirge of Urazya game:
Large mech (500 lb.)
Creature Capacity 1 Medium creature
Armor Class 22 (19 while motionless)
Hit Points 60 (damage threshold 10, mishap threshold 20)
Speed 60 ft.
STR 20 (+5) DEX 16 (+3) CON 16 (+3) INT 0 WIS 0 CHA 0
Damage Immunities fire, poison, psychic
Condition Immunities blinded, charmed, deafened, frightened, paralyzed, petrified, poisoned, stunned, unconscious
Mech. The creature piloting a mech benefits from having full cover.
Magic Weapons. The Hellmech’s weapon attacks are magical.
Jump. If the Hellmech moves at least 30 feet in a straight line, it can clear a distance of up to 30 feet when jumping over a chasm, ravine, or other gap. Each foot it clears on the jump costs a foot of movement.
Go! Drive and steer the Hellmech up to its full speed and take the Helm bonus actions.
Bolter Cannon. The Hellmech may move up to 30 ft and make one ranged attack. Ammunition: 50 bolter shells. Ranged Weapon Attack: +8 to hit, range 240 ft., one target. Hit: 14 (2d10 + 3) piercing damage.
Bolter Barrage. The Hellmech makes a ranged Bolter Rifle attack against any number of creatures within 10 feet of a point you can see within the weapon's range. It must have ammunition for each target, as normal, and you make a separate attack roll for each target.
Drill Fist. The Hellmech may move up to 30 ft and make one melee attack. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 16 (2d10 + 5) piercing damage.
Drilling Flurry. The Hellmech makes two Iron Drill attacks.
Juke. If the Hellmech is able to move, the driver can use its reaction to grant the Hellmech advantage on a Dexterity saving throw.
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I had to kludge the "Action Stations" rules a bit because a mech suit only really has one "action station" and it should be able to move and attack in the same round. So I built limited amounts of movement into some of the attacks.
I think you could swap the weapons I added above for any of the optional ones in Descent into Avernus. Flame throwers and such seem like easy adaptations. I stole the bolter barrage ability from a hunter ranger feature.
I think the rules could to extrapolated on to make even bigger mechs if needed. There's a gargantuan infernal war machine I would use as a basis for a really big mech.
Also note that by default the skill of the person using the weapons on an infernal war machine doesn't matter. The weapons have their own ability score mods and proficiency bonus, so piloting one of these is like stepping into a second character in a way.
By default, infernal war machines run on "soul coins." In Urazya, I think I'd make that a liquid fuel...but it would be interesting to keep the idea that it is created with souls. That sounds like the kind of technology that artificer vampires would come up with. Drink their blood as food, use their souls as fuel...use every part of the animal, so to speak.