Wanna fight a super powerful king?
How about his wyvern?
Okay. Now how about both at the same time?
Blasting you with lightning spears while his steed bites at you and rains down fire from its maw, this guy is considered the toughest boss in all of Dark Souls 3 by a lot of folks. Just keeping your eye on them is a challenge in and of itself as they fly around you.
And once you manage to drop the beast, the King himself is still a deadly threat. He's both fast and strong with erratic movements that can be hard to predict. His lightning blast and lunge are both particularly powerful and can easily kill an unprepared player.
Just make sure you're equipped with lightning resistant items.