The One Reborn
The street below Yahar'gul Chapel is dangerous, but you can reach Advent Plaza if you fight well. Your prize for making it there? An encounter with the hideous One Reborn.
The One Reborn is a very intimidating boss, but the real problem here is the band of witches that rain fireballs down from above - climb the staircases in the back corners of the plaza to reach them and take them out before you do anything else. As for the One, he'll stomp with his many legs up close; at a medium distance, he'll swipe at you from his front half. Your best bet is to let him attack first, dodge, and get in a few hits. If he collapses in a stunned state, use the opportunity to whale on him.
Another thing to watch for is his a blood spell. At close range (even when the One is stunned), blood and bones will rain from dark clouds that appear above him. Watch for him to conjure up some magic above his head - that's the cue for this move. In addition, be ready to retreat if the front part of his body starts glowing brightly, as it's about to unleash a damaging area-of-effect blast. Finally, his back end may spew acid all over the ground, so hurry to where the front half is facing to avoid too much damage.