The expression "was master of" usually implies control or dominance over something. Some synonyms for this phrase could include "had command over," "possessed authority," "held sway," "exercised dominion," "ruled over," or "was in charge of." These phrases convey a sense of power and control, whether it's over a group of people, a particular situation, or a broader concept. Depending on the context, other synonyms could include "dominated," "overpowered," "supervised," "governed," or "directed." By using these various synonyms, you can add nuance and specificity to your writing or speech and make your language more compelling and vivid.