Short answers, pulled from the story.
Eusebius of Caesarea was a historian of Christianity, biblical exegete, and Christian polemicist from the Roman province of Syria Palaestina. Born around AD 260, he became Bishop of Caesarea Palaestina soon after 313 and is regarded as one of the most learned Christians of late antiquity.
Eusebius earned the title Father of Church History because he produced the Ecclesiastical History, a chronologically ordered account of the Christian Church from the Apostolic Age to his own time. He began the work in the 290s and finished its first edition before the year 300.
Eusebius of Caesarea died on the 30th of May AD 339. Most scholars date his birth to some point between AD 260 and 265, most likely in or around Caesarea Maritima.
Eusebius wrote that Constantine described seeing a cross-shaped trophy formed from light above the sun at midday, bearing the phrase by this conquer, rendered in Latin as in hoc signo vinces. Eusebius did not date this to October 312 and did not connect it with the Battle of the Milvian Bridge, unlike Lactantius and later Christian tradition.
The works of Eusebius are valued largely for the copious quotations they preserve from sources now otherwise lost. His Preparation for the Gospel alone preserves Pyrrho's rendering of the Buddhist three marks of existence, a summary of the Phoenician priest Sanchuniathon, and material from Diodorus Siculus and the neo-Platonist Atticus.
Jacob Burckhardt called Eusebius the first thoroughly dishonest historian of antiquity, and Edward Gibbon distrusted his accounts of martyrs, citing a passage where Eusebius represents Plato as endorsing falsehood as a remedy. Defenders such as Joseph Barber Lightfoot argued that Eusebius's openness about what he omitted and his handling of testable quotations vindicate him against the charge.