can be found in Southern Foul Hills (2259, 1807). He looks like the Baby-Face mobs you might have encountered in higher level missions. You'll need at least two teams of level 180+ to take this mob down. It is probable that you will face up to five targets.
There are 4 Followers of The One (level 180-185) which guard The One (level 200) or it's placeholder Harbringer of The One (level 185-190). They all look the same, so be sure to target the right one. Be sure to pull only the boss or it's placeholder away from the followers to make it an easier fight.
Followers of The One Harbringer of The One
If you don't see The One, then it's placeholder Harbinger of The One will be there, he must be killed for a chance of The One to spawn in it's place. Harbinger will be an easier kill compared to The One but both do hit hard, so don't get too confident. You may have to kill the placeholder a few times before the main target spawns. Both also have the ability to warp players back to them, which means that if you're keeping your distance and all of a sudden you are out of combat be ready to run because a second later you will be next to the mob.
I must warn you that a team alone unless they are level 210+ will not be able to handle The One, because as soon as he is attacked 3-4 mobs called Pet of The One will spawn and go after anyone attacking their master.
Pet of The One
These little worm looking things may seem as no danger, but trust me they can be a pain. They hit as hard as The One himself (1-2k damage), they have a nano that will slow every action you make (running, nano casting, etc). Killing them won't work because a new one will spawn as long as you continue to attack The One. The best strategy is to root the pets and get them away from the teams while they continue their assault on the main target because he has a large amount of HP and he regenerates his health pretty fast.
The One drops several very, very nice items for those who do not have either the Shadowlands or Alien Invasion expansions.