Lost Ark is a side mission in Stellar Blade where you must solve a puzzle to find the password that opens the gate to the Ark. The mission is given to you by Orcal's bodyguard, Mann, who asks you to find a missing rebel.
Solution to Lost Ark Password Puzzle – Find the Ark Puzzle | Stellar Blade
The Lost Ark mission is unlocked after you return the second Hyper Cell to the Presence Chamber. However, you'll need to go to the Presence Chamber to talk to Mann, and there are certain points in the Stellar Blade story where the Presence Chamber is inaccessible. So you may have to complete a few story objectives before you can get there and talk to Mann.
Anyway, after you talk to him about the Radical, your first task is to find the rebel in Xion's Alley.
Exit the Presence Chamber and head directly across Memory Square (the square outside the Presence Chamber), up the stairs, and into the narrow alley. Around the corner lies a corpse. Search it to obtain the promotional document "Introducing the Ark-Tech's Ark!", which describes a safe haven available to citizens of Xion. Your next objective is to investigate this so-called Ark.
