Using the Lava Valve on the Adamantine Forge for the first time will release not only lava, but also Grym – The Eternal Protector of the Forge who’s an Adamantine Golem. Grym’s default defense and resistances are very high, so it will be a tough boss battle: this guide will offer you some tips on how to defeat him much more easily.
Get Grym into lava to cause Superheated debuff and make him more susceptible to damage.
Use Bludgeoning weapons to deal double damage in debuffed state.
Utilize Grym’s Vengeful Guardian feature to make sure Grym goes after characters who can withstand his blows.
Get the Myconid Bliss Spores buff on one of your warriors for more hits and damage (avoid rest to keep it).
Use spellcasters to cast Haste on those in close combat, from a distance (to avoid AoE attacks).
Keep someone by the Lava Valve to periodically reapply the debuff on Grym.
Grym Combat Strategy
This is a proper boss fight where you’ll have to be mindful of mechanics if you want to win, as Grym is pretty much indestructible in his default state. However, when he’s standing in lava, he gets the Superheated debuff that makes him vulnerable to Bludgeoning damage (meaning that Bludgeoning weapons deal double damage to him) and merely resistant instead of immune to certain other damage types.
Still, with Grym’s health pool of 450 and Armor Class of 18, you’ll want every hit to matter, so it’s highly advisable for all your close-quarters characters to equip a good Bludgeoning weapon. To reliably hit Grym, it’s also a good idea to get the Myconid Bliss Spores buff on one of your warriors (and make sure to reach the Forge without resting), as the extra 1d6 on Attack Rolls is really invaluable in this fight.
The role of spellcasters in this fight will mainly be to cast Haste on your warriors and stay far away from Grym to maintain their concentration as he has some nasty Area of Effect attacks at his disposal.
Grym is a very tough foe, but due to his Vengeful Guardian feature that makes him go after whoever attacked him last, you can abuse his AI to make things easier for yourself. Periodically throughout the fight, the lava will subside, so make sure you always have someone by the Lava Valve to reapply the Superheated debuff.
Once Grym is defeated, you’ll get the Grymskull Helm – a great piece of gear for your Heavy Armour users – and be able to complete your first round of Adamantine crafting. Should you decide to craft more than one piece of Adamantine gear, fret not – you won’t have to fight Grym again.