If you feel stuck on the main story quest Beyond the Gates, Defeat the Unhallowed Emissary you are not alone and lucky for you it’s not a bug! Although some feel like the Defeat the Unhallowed Emissary is bugged I can confirm its not, I too ran into this issue in New World and I was doing two things wrong. Location and timing.
First off, you need to be in the right location. He will spawn right before the final stair case up to the giant corrupted portal. (image above). Second, you need to have some patience, I have found that he takes a bit of time to spawn back. I ended up waiting about 25 minutes, but I was ALSO standing in the wrong location, so I might have just missed him.
Here are a few more photos so you can see the layout of the zone/where he will spawn. I hope this helps!