You could give an iron golem a flametongue shortsword or scimitar and have it just stab itself repeatedly for 1d6-1 damage and 2d6 healing. Who needs short rests anyway?

I had to fight 3 of these at the same time 

Quote from awer >>

but then you could never change back until the spell ended which is one hour

 True Polymorph allows you to change back whenever the caster wants. When the hour ends, the change is made permanent

definetly

Quote from FiatMcNuggets >>

Is it just me or does this guy look like something straight outta 40k?

 totally

Alchemists fire = 1d4 heals every round

Even better, swallow a fire elemental.
2d10 healing each turn from touching it, 4d6 + 6 healing from it attacking you. 

Quote from AlsoHeededShadow >>

What happens to a golem that has its master die...?

 it will continue to follow the its last order 

Does it swing it's arms and fling people? It should. Is it made with a pumpkin? It should be.

Quote from SkyOc1 >>

Combine this with one or more magma mephits to cast heat metal on this.

 I'm definitely using this. Great idea!

Quote from ExplosiveCarnage >>

Quote from DIemortalscum>>

It would be good for exploration though, you could explore a volcano or the plane of fire with ease. 

  But then wouldn't you be better off becoming a Phoenix? They have more mobility and can go through gaps at wide as 1 inch without squeezing. That seems better suited for exploring fiery places.

 Maybe, but depending on the situation, the Golem's longer list of immunities might be better for avoiding concentration checks. Though the Phoenix has a higher Con save so there is that. 

For a fun tweak, give it some type of fire or forge on its chest...dwarven made perhaps? 

You can add some extra HP and +1-2AC for flavor needing added materials to build the "forge" in the golem...your call.

Then add in this: After poison breath, the poison acts as a standing cloud the last 1d4 rounds. The golem can also then use fire breath (recharge 5-6) and ignite the  cloud causing 8d8 fire damage and making the spell fire wall in a 20x20 radius for the same 1d4 rounds. Now that makes it more of a CR 16 monster!!!!  Furnace/Bellows Iron Golem

How do you make one of these?

In my campaign my players are going to fight both Cthulhu and the kraken at about the same time, they'll kill the kraken then whilst sailing home cthulhu is gonna come wreck their world, they'll be about lvl 15 i am pretty sure (party of 5), so would befriending one of these be helpful or too op? they also will be fighting a dragon after getting back to land (very difficult journey, ik, but they like defeating overwhelmingly foreboding enemies for some reason?) so would it be too much to add one of these as a companion? i know i'm the dm and i get the last say but it would be helpful to get some advice.

Where will we get a big enough pumpkin to make this thing?

i thought of a cool concept: a flameskull who created an iron golem before it died, it still controls the golem via verbal commands and it can heal it by using fire ray on its turn

This is swol chonk boi.

Big funny did laugh

Quote from NiceDice69 >>

In my campaign my players are going to fight both Cthulhu and the kraken at about the same time, they'll kill the kraken then whilst sailing home cthulhu is gonna come wreck their world, they'll be about lvl 15 i am pretty sure (party of 5), so would befriending one of these be helpful or too op? they also will be fighting a dragon after getting back to land (very difficult journey, ik, but they like defeating overwhelmingly foreboding enemies for some reason?) so would it be too much to add one of these as a companion? i know i'm the dm and i get the last say but it would be helpful to get some advice.

 well unless they make the golem or the golem has some sort of amulet that controls it then iron golems only follow the orders of there Creator also they have very low INT so they can make decisions on there own

is grease am magical effect save?