(born Aug. 31, AD 12, Antium, Latiumdied Jan. 24, 41, Rome) Roman emperor (37–41). Known by his childhood nickname, Caligula (Little Boot) was declared heir to the throne by Tiberius following the suspicious deaths of Caligula's parents and brothers and probably connived in Tiberius's death. Caligula suffered a severe illness seven months into his rule and began displaying mental instability, engaging in despotic caprice and cruelty. Restoring treason trials (38), he executed former supporters and extorted money from the citizens. He plundered Gaul in 40 and began planning to invade Britain. He made pretensions to divinity and declared his sister Drusilla a goddess on her death. Weary of his tyranny, a group of conspirators assassinated him.