Hi all,
I am currently running my players through the Paizo Adventure Path Iron Gods (think a reimagined and expanded Expidition to the Barrier Peaks). I am converting it to 5e as I go and just redid the final boss for the adventure who is a 3.5e CR8 AI/Robot with quasi-divine abilities. I have attached my conversion below and I am concerned (at this stage at least) he may overwhelm my players' characters. He will be up against six level 8 characters by the end of the adventure. I used the DMG guidelines for creating him from scratch with the original 3.5 version as inspiration. I wondered what more experienced DM's thought and if they felt he would be too deadly. I do want him to be a signficant and memorable threat though:
HELLION’S CHASSIS
Large construct (robot, mythic, aggregate), chaotic evil
Armour Class 16 (heavily reinforced skymetal chassis)
Hit Points 190 (19d10, maximized)
Speed 30’
Str 22 (+6) Dex 10 (+0) Con 10 (+0)
Int 14 (+2) Wis 17 (+3) Cha 19 (+4)
Skills Athletics +4 (+6 when jumping), Persuasion +7, Intimidate +8, Knowledge (engineering) +7, Knowledge (religion) +5, Perception +8, Insight +6
Damage Resistances bludgeoning, piercing, and slashing from nonmagical weapons that aren’t adamantine.
Damage Immunities force, necrotic, poison, psychic
Condition Immunities lightening, charmed, frightened, paralyzed, petrified, poisoned, stunned
Senses Darkvision 60 ft., passive Perception 13
Languages Gothic, Common, Orc
Challenge 8 (3,900 XP)
Amazing Initiative. Hellion has advantage on initiative rolls and due to his dual nature (multi-processing power) also has two initiative counts and may act on both counts. Hellion usually uses one initiative count to attack with its inhabited robot and another count to cast a spell.
Explode. If Hellion’s chassis is reduced to 0 hit points, it explodes, dealing 16 (8d4) points of plasma damage (half fire, half electricity) in a 20-foot-radius sphere (Dexterity DC 16 half, normally 4 targets).
Mythic Power. Hellion can draw upon a wellspring of power to accomplish amazing deeds and cheat fate. Hellion has 4 uses per day of mythic power and can use this ability as a surge or to take a bonus action. When Hellion uses it to surge he may add 1d8 to any d20 he just made adding it to the roll. This can change the outcome of the roll.
Spawn of Unity. As a creature of technology created by Unity, Hellion has inherited many divine abilities and technological immunities and resistances.
Innate Spellcasting. Hellion’s innate spellcasting ability is Charisma (spell save DC 16, +7 spell attack). Hellion can innately cast the following spells once per day, requiring no material components:
1/day each: bane (C, 3 creatures, CHA save, -1d4 from attack/save), blindness/deafness, command (1r, WIS save, grovel), false life (+1d4+4 hp), hellish rebuke (reaction on damage, DEX or 2d10 fire), inflict wounds (melee 3d10 necrotic), moonstruckAPG (confused, enraged and gain natural attacks), protection from good (C), ray of enfeeblement (C, 60’ range, half-damage on STR based attacks), ray of sickness (2d8 poison, CON save or also poisoned until end of my next turn).
LAIR ACTIONS
On initiative count 20 (losing initiative ties), Hellion takes a lair action to cause one of the following effects; Hellion can't use the same effect two rounds in a row:
· Automated Boltor Cannons pop out of cunningly hidden recesses in the roof and spray the area with gunfire. They are superhumanly accurate and only hit enemy creatures. Each creature in the room must make a DC 15 Dexterity save or take 12 (4d6) piercing damage.
· Hellion floods the room with tear gas. All breathing creatures must make a DC 15 Constitution saving throw or spend the round heaving and retching. Effect ends after a successful save.
ACTIONS
Multiattack. Hellion makes three attacks: two with its claw and one with its plasma beam. Or three plasma beam attacks.
Claw. Melee Weapon Attack: +9 to hit, reach 10ft., one target.
Hit: 14 (2d8 + 6) bludgeoning damage. If the target is Large or smaller, it is grappled (escape DC 16). Until this grapple ends, the target is restrained; Hellion automatically does 14 (2d8 + 6) damage until the grapple is broken. While crushing an opponent Hellion losses a claw attack.
Plasma Beam. Ranged Weapon Attack: +3 to hit, range 50/100 ft.; one target.
Hit: 22 (5d8 + 2) plasma damage.
Apologies, I cant seem to remove the fake png links for some reason 
While I am not sure how difficult this will be for the players, I did notice something on the spell casting. The Save DC is 8+PB+Cha mod. It should be DC: 15. The Spell attack modifier is correct.
That monster looks pretty hardcore. I am planning on picking up that adventure path after they release the last one in PDF this month. I already bought the Technology Guide.
Hi Torg, good point. I was just using the DMG guidelines but yeah, I was being lazy there, will change it. I will let the characters get items to nuliffy some of his attacks like the tear gas.
You can have them find Filter Masks while they are adventuring. http://paizo.com/pathfinderRPG/prd/technologyGuide/gear.html#filter-mask
Yep, going to add some filter-plugs/masks at some point. Plus they have an opporutnity to give the AI a headache but essentially releasing a virus to weaken it somewhat. This is assuming the characters figure it out 
A CR8 Monster vs 6 Level 8 PCs won´t stand a chance.
Following the guidelines, he is worth 3900xp * 0,5=1950 xp (as he is a single monster vs 6 PCs) Divided by 6 and you are left with 325xp per character. That is bareley an easy fight.
You need to either add a second one of them or at least add some advantageous terrain. Maybe minions.
A good way to improve him would be adding legendary resistances and actions. And still, vs 6 Persons, legendary resistances protect it against maybe one round of spells.
On the other hand, comparing him to a CR8 monster you should notice, that his offensive is way higher, as he has 2 initative counts. Using his attacks, he does 50 damage per round if they all hit (which is assumed in the calcuation)
Thanks UngeheuerLich. I thought he maybe too strong, I'll play it by ear for now. Players are still a while away before they confront him, if they deal with his minions too easily I'll add more power.
The discussion (and experience) with the Mutant Manticore inspired me to rework Hellion into a more appropriate CR 14 mob with staged boss fight type mechanics. For those interested. Raid boss erdana, Arial, Helvetica, sans-serif">Hellion is totally untested. My characters won't face him for a few sessions yet. So use at your own risk. To me he looks fine on paper.
I ran the Hellion 2.0 boss fight last night. Worked well over all but my thoughts, expereince and actual mechanics are here.