Short answers, pulled from the story.
The name Croat derives from the Proto-Slavic term Xъrvátъ, which likely traces back to a 3rd-century Scytho-Sarmatian form found on the Tanais Tablets. Archaeological evidence suggests that early Slavic migrations into Southeastern Europe occurred during the late 6th and early 7th centuries.
King Tomislav became the first crowned ruler of Croatia around 925, uniting Pannonian and Dalmatian territories under one state. His army reportedly included up to 100,000 infantrymen and thousands of cavalry units according to Byzantine accounts.
Croatian nobles elected Ferdinand I Habsburg as their new king on the 1st of January 1527 during the Cetin election. The empire organized border regions into a Military Frontier directly ruled from Vienna military headquarters.
A majority of male Croats belong to haplogroups I2 and R1a which relate to early medieval Slavic migrations from Ukraine and Southeastern Poland. Autosomal studies show shared ancestry between Croats and populations in Romania-Bulgaria clusters dating back approximately 1,500 years.
The Baška tablet from the late 11th century contains text written mostly in Chakavian dialect using angular Glagolitic script. It mentions King Zvonimir who ruled at the time and serves as an early record of the language.