The pillar at the center of the Sect still retained its ability to extract souls, just like any of the smaller pillars scattered around the city. Tany and Azerion investigated the base of the pillar underground, asking whether there were any records concerning this place and its original purpose.
The Rosvelt Sect allowed them to access records that couldn't implicate the Sect in any way, pretty much similar to how the Thousand-Script Sect had allowed Thery's group to examine certain information. As long as the records didn't contain anything that could threaten Rosvelt's interests, there was little reason for them to hide them.
That's how they found out that the place where the pillar's base was located was called the Woster Guarding Grounds.
