A massive cache of hacked e-mails and leaked documents from some of Russia’s most prominent and shadowy power brokers are expected to be published online by a group calling itself Distributed Denial of Secrets (DDOS), which claims to be “aimed at enabling the free transmission of data in the public interest.”

Emma Best, a US-born journalist and co-founded of the group, said the materials would include various archives of hacked and leaked materials related to Russia and various high-level Kreml...