The Vespers are an extremely dangerous organization based in Germany.
The Vespers were a family, dating back to the early 1500s, just like the Cahills. They evolved into an organization, and, according to George S. Patton, they recruit the best in the world and aren't picky apart from that the recruits need to have talent. Damien Vesper, the first Vesper, was Gideon Cahill's best friend until he betrayed him for the Master Serum, forcing Gideon to start the fire which burned down Gideon's laboratory. It is known that they are the Cahills' worst enemy. The Vespers have been sabotaging the Cahills from the start and framed Buchanan Holt for giving the Ekats the secret message. In the introduction to the The Black Book of Buried Secrets, Rick Riordan stated that agents (i.e. Margaret Peterson Haddix, Jude Watson, Patrick Carman, etc.) present during their meeting seemed to be terrified by the mere mention of the Vespers. They seem to be worse than any Cahill. As stated by William McIntyre, "They make Isabel Kabra look like Mother Teresa," and that from the reader's standpoint, Isabel is generally considered the worst, cruelest, and most ruthless Cahill.
The Vespers have also intruded and had double-cross agents all over the branches, making them more powerful and have more information that the Cahills.