The Vault Keeper is the main boss in Season 3 of Diablo 4, but that doesn’t mean the Son of Malphas can’t show up to protect his demon dad. If you want an extra battle with some decent loot, I’ll go over where you can find the optional boss in the Season of the Construct.
Where to Find the Son of Malphas in Diablo 4
The Son of Malphas can appear as a random boss encounter in one of the five Nightmare Vaults on World Tier 3 or higher. Nightmare Vaults can only be accessed after reaching World Tier 3 and they serve as the upgraded version of Vaults from the main quest line in Season of the Construct. Any time you enter one of the higher-level Vaults, the offspring demon can appear.
Much like the Butcher from standard dungeons around Sanctuary, the Son of Malphas is completely based on chance. You can be in the middle of completing the Vault of Stone, and if you see the boss bar appear, you know what to expect now. This boss completely replaces the Butcher in Vaults, and much like the standard version, losing the battle means you lose your chance for loot.
Even after you defeat this optional boss, it can continue to appear in later Vaults. By the time I was level 65, I had faced off against the Son of Malphas three different times. After each encounter, I was able to loot some Legendary equipment and Tuning Stones to help upgrade the seasonal mechanic.
How to Beat the Son of Malphas in Diablo 4
The Son of Malphas alternates between a jump attack and a fireball cone that needs to be dodged if you want to defeat him. Most of the time you can heal through his attacks, but once he starts using the fire sword cone, you want to stay on his back. Shortly after the most devastating cone this boss has, he will disappear in the air.
Within a couple of seconds, you will see a circle appear on the ground. That circle is his landing spot and you need to dodge right away. From here you can get back to dishing damage and get ready for the cone attack all over again.
After dealing with the son, make your way to the Echo of Malphas in Diablo 4.