To obtain the Old Friends side quest, talk to Arish when he's in Central Executive.
Defeat Hiss-corrupted Guy
First up in the Old Friends quest is Hiss-corrupted Guy, over in Maintenance - Astral Chamber. Fast travel there and he'll spawn in front of you instantly. There's nothing particularly special about him, he's just a standard Hiss Distorted. Use everything in your arsenal to take him down and get the pouch for Arish.
Defeat Hiss-corrupted Hazzard
To get to Hazzard, you can't fast travel directly there so from the Astral Chamber, go up the stairs to the east and through the room with all the friendly Rangers, refusing to give you a hand. Thanks you guys.
Keep running south and you'll find Hazzard, who is being shot at by some much more helpful Rangers. He's even easier to defeat than Guy, so when he's down, sprint back to the Astral Chamber to fast travel.
Defeat Hiss-corrupted Cho
Cho is in Ventilation and you can fast travel straight there, just head through the corridor to the larger area to find him. Again, there's some useful Rangers here helping you out, but Cho also has a Hiss Cluster healing him. Take that out then Cho should go down like a piece of cake.
Defeat Hiss-corrupted Thomson
Head to the Black Rock Quarry Entrance next because that's where Thomson is hiding out, along with his Astral Entity buddies. There'll be some more Rangers here lending a hand, and Thomson should go down quickly. Just make sure you keep stunning the Entities.
Defeat Hiss-corrupted Heptonstall
Fast travel to the NSC Coolant Pumps Entrance, then follow the signs for the Coolant Pumps. Heptonstall is a little different to the others because he's been turned into a Hiss Distorted. You just need to look for the black smoke floating through the room and when you spot it, use Launch to break Heptonstall's disguise and he'll be incredibly weak.
Defeat Hiss-corrupted Remus
Finally, go to the NSC Energy Converters Entrance for Remus. He's a Hiss Elevated, so use your pistol to hit him a few times then when he's stunned, Launch will connect as he can't dodge.
Speak with Arish
Now you just need to head back to Central Executive to speak with Arish again to complete the mission.
Main Missions:
Control walkthrough: Welcome to the Oldest House
Control walkthrough: Unknown Caller
Control walkthrough: Directorial Override
Control walkthrough: Old Boys' Club
Control walkthrough: Threshold
Control walkthrough: My Brother's Keeper
Control walkthrough: The Face of the Enemy
Control walkthrough: Finnish Tango
Control walkthrough: Polaris
Control walkthrough: Take Control
Side Missions:
Control walkthrough: Old Friends
Control walkthrough: Fridge Duty
Control walkthrough: Self-Reflection
Control walkthrough: Mr Tommasi
Control walkthrough: Old Growth
Control walkthrough: A Matter of Time
Control walkthrough: Langston's Runaways
Control walkthrough: The Enemy Within
Control walkthrough: Mold Removal
Control walkthrough: What a Mess (all missions)
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