Short answers, pulled from the story.
Theodosius I was Roman emperor from 379 to 395 and the last ruler to govern the entire Roman Empire. He was initially styled "the Great" to distinguish him from his grandson Theodosius II; the honorific was formally deemed merited at the Council of Chalcedon in 451 in recognition of his promotion of Nicene Christianity.
The Gothic War concluded on the 3rd of October 382 with a settlement allowing the Goths to settle Roman land south of the Danube in exchange for military service. Critically, the Goths retained autonomy under their own leaders and fought as a national contingent rather than being integrated into Roman forces, terms that modern historians regard as disadvantageous to the empire.
The Massacre of Thessalonica occurred in April 390, when Roman soldiers killed a large number of civilians, likely in response to an urban riot that had resulted in the death of a Roman official named Butheric. Scholars disagree on Theodosius's direct responsibility; some believe he ordered the massacre in anger, while others argue a planned selective killing got out of hand while the emperor was in Milan.
The Edict of Thessalonica, issued on the 28th of February 380 by Theodosius, Gratian, and Valentinian II, decreed that only Christians who accepted the consubstantiality of the Father, Son, and Holy Spirit could call themselves "catholic" and have their churches officially recognized. It is the first known secular Roman law to positively define a religious orthodoxy, though recent scholars note it was addressed only to Constantinople and largely went unnoticed outside the capital.
The Battle of the Frigidus, fought on the Vipava river on the 5th and the 6th of September 394, was Theodosius's victory over the western usurper Eugenius and his general Arbogast. On the second day, a natural wind phenomenon called the Bora, capable of hurricane-strength gusts, blew directly against Eugenius's forces; Eugenius was captured and executed, and Arbogast killed himself on the 8th of September.
The Obelisk of Theodosius is an ancient Egyptian obelisk originally from Karnak that Theodosius had transported to Constantinople and re-erected in the Hippodrome in 390. Its white marble base is covered in bas-reliefs of the imperial household and has been identified as the key monument of the Theodosian court style. It still stands in the former Hippodrome of Constantinople today.