There are many Emperors whom nearly nothing is known about. But the majority of these cases are Emperors whom only ruled for a few months, or were co-emperors who held little actual power Sometimes both, such as in the case of Gordian II, who's only noteworthy achievement is being the brother of Gordian I. Of noteworthy emperors, Emperor Trajan's long rule is surprisingly lacking in detail, and there are few primary sources about it. Most of what we know about Trajan's reign comes from Historians after his death, who are simply summarizing it. The lack of Primary sources is likely due to the last generation of Historians dying off shortly before(edit: Tacitus during Trajan's reign and Suetonius shortly after) Trajan's reign, and the next generation not beginning to write until after it. This being said, we still do know a lot about Trajan from summaries and monuments, particularly Trajan's column, which details his conquest of Dacia.