Before I begin: MAKE SURE YOU SAVE! Achievements have a way to glitch. Saving as a precaution before you're supposed to get the achievement helps you to try again without having to start over.
While it's not considered part of the main story, you will want to fight the Hekatonchires if you want to see the rest of the story.
The side quest chain starts with Most Adamantly, which you can start before the main quest ends. The next one you can't start until after it ends.
Anyway, here are the quests in the chain:
*** Spoiler - click to reveal ***Most Adamantly
Recover the Adamant without being detected.
Blood Gets in Your Eyes
Select "Getting to know each other", then "Don't talk about her like that."
Good From Far, Far Good
Pick any option, then "The Dikastes will help you!"
Sixth Sense of Sympathy
Talk to the harbormaster, playing along with her charade, then kill the enemies.
Through the Cracks
Fight through the enemies at the quarry until you reach the Foreman, then threaten him. Now, report back to Alkon and then kill Xarios at the Fortified Doma to end Good From Far, Far Good.
Deadly Little Secrets
Select "Humans aren't rats" and then you can choose to either save the humans (moral choice) or go through the door (evil choice). Save if you want to see both. I won't spoil what's behind the door but it's worth a laugh.
Now, prepare for the final mission. You'll want to equip poison resistance because hatchlings (I forget what they're called will spawn and explode if you're near them, dousing you with a poisoned mist. Make sure to enter the final battle with plenty of arrows. You don't want to get too close to the boss. It can (and likely will) pick you up and ragdoll you.
The Blessing of Kronos (Venomous Attacks enhancement) is great for keeping your distance while doing melee (even if you're using daggers). You want it for another achievement anyway.
Also, solve the Ostraka puzzles if you intend to use the Blessing of Kronos and Ares Madness. Complete all five to unlock +10% Damage with Enhanced abilities. Then go to Myson Cave and have Hephaistos upgrade it to +20%. You can then engrave it on your arms and legs for up to +40%. Finally, if you want to use the Ares Madness (Battlecry of Ares enhancement), you should equip the First Civilization legendary set.
Finally, you're ready for the final mission. SAVE NOW!
Fate of Alantis
Talk to Poseidon. Select Humans are being tortured," followed by "I don't belong here," and then "I'll stop the Hekatonchires."
If you're low on health, you can tactically rely on Overpower (there's an engraving that lets you have -1 adrenaline point for overpower attacks), or use Ares's Last Breath in combination with Blessing of Kronos. Another ability you can use tactically is Ares Madness. If you really need adrenaline, you can use Fury of the Bloodline. However, your best defense is to stay away from the boss. Once the boss is defeated, enjoy your achievement and the rest of the scenes.