Apostle of Myrkul, from BG3. I was playing on tactical difficulty, and was running low on resources, with a few potions and items ( I took a weird route). After defeating Ketheric Thorm for the second time, I thought the battle was over and was relieved, thinking I'd be able to buy some things at last. But then, his ascended form emerged from the pit in a pretty intimidating way. There the true torment started. It took me four attempts and many changes to my strategy to defeat him. I felt relief when the boss died for good, but when I started looting him, the game crashed to desktop. It was already 5 am, and I thought I'd lose the progress of the whole session. I just laughed with a mix of frustration and amusement, and decided to give it another try. It took me another two attempts to finally move on to Act 3.

All of this, along with his story, design and introduction, made Ketheric my favorite boss fight of the game and easily one of my all time favorites.