Titan Quest

This achievement isn't story related, nor is it unlocked via a side quest like the Ragnarok DLC. This is earned in the arena, which you can access from a portal in either Gadir northeast of the NPC Marinos (Atlantis starting town) or Corinth northwest of the teleport (Ragnarok starting town). In either case you need to talk to the Senechal of Tartarus near the portal so the portal opens. If you've started neither DLC your fastest access is going to Rhodes in act 4 and talking to Marinos east of the teleport by the beach, this will take you to Gadir.

There is 5 waves, with the 5th being the boss, Tartarus. The enemies scale to your level and some of the modifiers you can get are pretty nasty, so even on normal this can be a challenge. I can't really provide much in the way of tips, because I don't know your build is or what modifiers you'll get. I had to try a few times in order to avoid both really bad modifiers, and it also has a crashing problem. Sometimes when I would try to enter the arena the game would slow down to a slideshow, and progressing further just crashed the game, destroying my run. Anyway, I was able to make it with whatever build I was using back in 2018 when I first beat the main game. The Archery tree and Dream. I have no clue if it's actually any good, or good enough to beat Tartarus on legendary, but I got all the achieveable achievements with it. We'll probably never need to know though, since they probably won't fix the legendary achievement.

The one thing I can say is do not fight Tartarus in melee. He did some attack that did 14000 to my 4000 HP health pool on normal, and I would guess epic would have been worse. Maybe it was just his mace slam, or mayeb it was the modifiers. Either way if you stay at range the only thing you really need to worry about is him dropping ice, as if it hits and freezes you and you're near him, or some of his goon squad minions, you probably die and have to start over. Other than that he has a fire DoT that does decent damage on epic, and a lightning (?) wave that can do good damage as well. I had 80% resist to both so I stayed our of the fire and tanked the lightning and was ok. Assuming you aren't in danger of dying it's just blowing through his health pool in 5 minutes, which is a DPS check.