Demons Souls Flamelurker. He's a rare occurance in that he's actually not that large or highly-damaging for a boss, most kill you in one or two hits, Flamelurker averages 3 or 4. He makes up for it by being really, really fast. He's difficult to hit, has short tells before his own attacks, and the lower his health gets, the hotter this fire demon burns. At about the halfway point you start taking damage for simply being near him, so you have to be careful for multiple reasons. Now from Dark Souls, Artorius the Abysswalker. Like Flamelurker, he's fast more than strong, and doesn't flinch often, so you have to be extremely careful. However, where Flame gets to deal aura-based damage halfway through, Artorius has some attacks you, quite simply, have to dodge. They come out faster than you might expect, and are a nice mix-up if you're used to blocking for him.