Final answer:
The Plague of Justinian, caused by the bacterium Yersinia pestis, was the deadly pandemic that struck the Byzantine Empire in 542 AD, leading to massive population losses and economic decline.
Step-by-step explanation:
The "invisible killer" that arrived in the Byzantine Empire in 542 AD was the Plague of Justinian. This outbreak was caused by the bacterium Yersinia pestis, the same pathogen responsible for the Black Death centuries later. The plague resulted in the death of about half the population of Constantinople and one-third of the population of the entire Byzantine Empire, severely impacting military recruitment and morale. It also had long-term economic ramifications, contributing to a decline in long-distance trade and the fragmentation of economies.