On the island of Blackreef, time doesn’t progress normally. At the end of each day, the cycle begins anew, resetting everything to the way it was at the start of the day. Well, mostly everything.
If you infuse your weapons and abilities with residuum, you can keep hold of them when the day resets, even after you run out of reprise uses and are sent back to the start of the day.
You only acquire this ability after a certain point in the story - after you defeat Wenjie in The Complex, as part of the ‘Ubiquity’ mission.
You acquire residuum by interacting with glowing items, or by defeating Visionaries. If you’ve used reprise to rewind your death, you can also return to the location at which you died to pick up residuum there.
You can infuse items between each time period when you have access to the equipment screen. In the menu, select ‘loadout’, then ‘infuse gear’, and hover over the weapon or ability you wish to infuse.
Hold the ‘enter’ key to infuse them, locking them in for future loops. If you decide you no longer want to keep your infused item, you can delete it by clicking on the item and then holding down the delete button.
Be aware that you lose all the residuum you’re holding when you get sent back to the start of the loop, so make sure you spend it wisely.
There’s a safe in Updaam that contains 5,000 residuum, and once you figure out how to open it, you can return to it every loop and take its contents for yourself. Here’s how to figure out the code.