To finish step 2 of this codex, go to one of the blue circle markers at night. There should be two magic circles in that area: yellow and red (see the screenshots tab). Wait until enemies emerge from both circles and begin negotiation. Once they're talking to each other, kill them all and loot the crate to get the key.
It's important to wait until both parties are there and talking to each other. If you kill first party before another arrives, second party will not come at all and you'll have to go to another marker.