Ken Griffin has become impossible to ignore in the once-lucrative world of credit derivatives, where a group of Wall Street dealers long maintained a stranglehold. After being kept on the sidelines for the past eight years, Griffin’s Citadel Securities is muscling into the market. The firm has traded more than $116 billion of credit-default swaps tied to U.S. benchmark indexes since April, according to spokesman Zia Ahmed. That’s about 11.5 percent of total trading of those contracts when compared to reported data aggregated from the Depository Trust and Clearing Corp. and Bloomberg’s swap data repository.