Complex sentence: Later that same month, after a series of heart attacks over the previous two years, Golos died in bed in the arms of Elizabeth Bentley, who then took over operations (thus the reference in the decrpyts to him as a "former" colleague).
Split into simpler sentences: Later that same month, after a series of heart attacks over the previous two years, Golos died in bed in Bentley's arms. Bentley then took over his operations (thus the reference in the decrypts to him as a "former" colleague).