When Olivia Cahill died, she entrusted Madeleine with the ring of Damien Vesper, the founder of the Vespers and Gideon's former friend.

The friendship of Gideon Cahill and Damien Vesper broke apart when Damien suspected that Gideon recently discovered a formula that could make a man the most powerful human being in the human history.

The ring is only mentioned in passing in The Black Book of Buried Secrets, but it is very possible that it will play an important role in both Vespers Rising and Cahills vs. Vespers.