In 'Measure for Measure', the duke who pretends to leave Vienna but stays disguised as a friar is Vincentio.
Duke Vincentio announces that he must leave Vienna and appoints Angelo as his temporary deputy. In reality, Vincentio remains nearby in disguise as Friar Lodowick so he can observe Angelo's rule and learn how the city's neglected laws affect its citizens. His disguise lets him investigate without the authority and ceremony of the ducal court.
Angelo soon sentences Claudio to death for having sex outside marriage, then tries to exploit Claudio's sister Isabella in exchange for mercy. Vincentio, still disguised, advises Isabella and helps arrange the “bed trick,” in which Mariana takes Isabella's place when Angelo expects to meet Isabella. He also works to prevent Claudio's execution.
Near the end, Vincentio returns publicly as duke and exposes Angelo's abuses. The friar disguise is central to the play's themes of justice, surveillance, mercy, and the gap between public authority and private conduct. Escalus is an older lord, Claudio is the condemned young man, and Angelo is Vincentio's deputy.