The Smelter Demon is a Night Boss in Elden Ring Nightreign, which returns from Dark Souls 2.
This Boss swings its heavy sword around in telegraphed moves that can be easily dodged by rolling or running in circles around him. Melee characters should stay near the Demon and employ hit-and-run tactics. Ranged players should avoid standing right in front of the Boss to avoid his lunge attack.
When the Smelter Demon’s HP falls below a certain threshold, he will ignite himself and his sword on fire, causing his attacks to deal higher fire damage and continuously damaging nearby players due to the heat. His moveset becomes more aggressive and he’ll be able to jump and cleave his sword in the ground triggering explosions in a straight line.
The keys to this fight are having some sort of fire resistance, reading his slow and telegraphed animations, and keeping an eye out for your own HP when continuously standing nearby to avoid being K.O’d by the heat damage. This boss is, however, considerably easier than other Night Bosses, so parties shouldn’t have too much trouble dealing with him.