I did find this in the SP milestones, but there is something that can cause the icon to pop far before that point. The first time I saw it, I decided to farm the tier-R stuff before entering.
The trigger for this is: hitting 50 souls for the first time in an infinity opens Abyss D. Getting the 51st soul closes it. Getting 50 souls again after a rebirth does not retrigger this.
