Final answer:
The question about who directly challenged Maria of Antioch's power is not addressed in the resources provided. Instead, the Byzantine Empire faced threats from Bayezid I and Timur externally, and the powerful dynatoi elite internally.
Step-by-step explanation:
The individual who challenged Maria of Antioch's power in Byzantium was not specified in the resources provided. However, during the time periods discussed, the Byzantine Empire faced numerous challenges both from external and internal adversaries.
Externally, figures like Bayezid I and the forces under Mongol conqueror Timur attempted to capture Constantinople.
Internally the dynatoi, powerful Byzantine elite families, often sought to leverage weaknesses in the empire's governance to advance their interests. emperors following Basil II, who attempted to curb the power of these elites, were less effective, leading to a reliance on the dynatoi for support and ultimately weakening the empire's structure.