The Fire Fiend is a miniboss which is spawned by solving the Dungeon Puzzle. Its body is very hot and any entity that gets close to him is quickly hurt from fire damage.
The Fire Fiend spawns below the dungeon structure and quickly breaks his way up to the surface. Then it starts rapidly shooting fireballs at players below, and once a player gets on the surface close to the Fire Fiend, it will go to the next stage.
Fire Fiend's main attack is fireball spawning. It chooses a player and starts forming fireballs around that player, using fire particles. After a few seconds, fire particles start turning into actual fireballs and are shot at the target.
When all fireballs are shot, the Fire Fiend has a chance of doing his second attack, during which it gets very close to the target and uses its passive attack to deal fire damage to the target. Once enough damage has been dealt, the attack stops. Then the Fire Fiend waits several seconds before next fireball attack.
Blast resistance and Fire resistance are very important against fireballs. Although the fireballs can be reflected just like Ghast fireballs, they are shot very quickly and getting hit by some of them is inevitable. Powerful bow is best weapon, because the Fire Fiend will always stay above ground, and the only time you can use melee is when it gets close to you during the special attack.