Short answers, pulled from the story.
Maxentius was acclaimed emperor by Rome's city garrison on the 28th of October 306. Officers named Marcellianus, Marcellus, and Lucianus offered him the imperial purple after he had been passed over when Diocletian and Maximian abdicated in 305.
Despite holding Rome and receiving recognition from the Senate, Maxentius was never formally acknowledged by the other emperors of the Tetrarchy. Galerius, his own father-in-law, refused recognition partly from personal antagonism and partly to deter others from seizing thrones unilaterally. He was called a usurper by every other emperor throughout his reign.
Maxentius built the Basilica of Maxentius in the forum Romanum, later completed and credited to Constantine, the Temple of the Divine Romulus dedicated to his son Valerius Romulus, and the Circus of Maxentius along the Via Appia. He was the last emperor to permanently reside in Rome, and many buildings commonly associated with Constantine were in fact his projects.
Maxentius died on the 28th of October 312, when his army was defeated by Constantine on the Via Flaminia north of Rome. As his troops retreated toward the Tiber, Maxentius fell into the water and drowned. His body was recovered the following day, paraded through Rome, and sent to Africa as proof of his death.
Maxentius ended the Diocletianic Persecution in his territories at least five years before the Edict of Milan, based on documents identified at the Biblioteca Marciana in Venice in studies conducted between 2021 and 2022. His first epistle made Christianity a lawful religion and abolished the existing edict of persecution by law. He also allowed the Christians of Italy to elect a new Bishop of Rome, Eusebius.
In December 2006, archaeologists excavating near Rome's Palatine Hill unearthed wooden boxes containing three complete lances, four javelins, a base for standards, three glass and chalcedony spheres, and an elaborate sceptre holding a blue-green globe. Archaeologist Clementina Panella identified these as the only known imperial regalia ever recovered from the ancient world, previously documented only through coins and relief sculptures. The items have been restored and are on temporary display at the Museo Nazionale Romano at the Palazzo Massimo alle Terme.