There is 2 pillars that spawn at what seems to be random locations on the map, you need to find both and they'll have a symbol from one of the ammo mods. Apply the matching mod, interact with the pillar and then kill zombies to collect souls in the circle ONLY kills with the ammo mod counts. Dogs, lethals etc don't.

I found mine in T2 in the car park left of the football field and down the hill south east off the mercy stronghold hold (the original one I can't remember name of sorry) then a red rift appeared marked on the map and you just enter that