The Soul of Fire is a new soul in Avalon. It is dropped by Flame Bats which drop 1 in rare intervals, the Wall of Flesh in quantities of 3-5, and from the Dragon Lord in quantities of 15-17. It is required in the crafting of Dragon Lord Armor and some other fire-related items. It also is rarely dropped by an underworld NPC in Superhardmode. It appears that this soul is only available in Superhardmode. Also worth noting that the Wall of Flesh may not drop Souls of Fire. This is a known bug. One way to fix this is to open NPC.cs and go to line 163 and remove the /* at the start, and the */ at the end.