Seeking "grimdark" book. Unknown author and title. I remember there is a burned man type character who seals himself in barrels of sewage or vinegar who later emerges and seeks revenge by shooting flames. Read the book around 2016-2018

[Colten]

Additionally there is a young boy learning magic that involves using candles and projecting and magnifying the heat or something. I also remember a scene later in the book where the boy is using this magic to burn ships that are attacking the castle or area he is in.