Use Cabal Gold to socket a Challenger Medallion into the Hammer of Proving. Once done, you will be ready to issue a challenge. The Hammer of Proving can be found in the Quest tab of the Director.
Caiatl's emissaries flood Sol with ceremonial Gold, a custom during the Era of Conquest. Conquered worlds are enriched to ease defeat and afford the opportunity to challenge officers for representation within the Cabal's leadership.