To get the Hellfire Power Armor, you must dispatch of Pyro to claim the set for yourself in Fallout 4 ... which rewards you with Enclave-themed content, and Speak of the Devil, another quest ...

Clear them out and you can loot the Enclave Hellfire ... Armor. You can now complete the trial and walk on the magma to collect the Legendary Weapon. Follow this quest to acquire the X-02 Mk. 3 ...