Dammon in Baldur's Gate 3

Dammon is a tiefling blacksmith who is originally located inside the Druid Grove, where most of the tieflings are (the cave where Will trains children). Talk to him. He needs hellish iron. When you find everything, you can create infernal armor.

Dammon may not be able to make all the armor at once, so you will have to meet him in the second and third acts. In the second act, he is in a barn next to the Last Light tavern. In the third act, look for him in Baldur's Gate.