[drran]

Historical region called Russia (Русся), but suffix -sia (-sia) was dropped in Slavic language, because it means this/there: Russia (Русся) -> Rus` sia (Русь ся), Rus` there.

Russia founded by descendants of Russia town, now called Old Russa (Старая Русса), which left their town due to epidemic of plague of Justinian.

[ceilingcorner]

Russia didn't exist until nearly a thousand years after the plague of Justinian. Unless you just mean the plague?

In either case, Rus comes from the Scandinavians, not from the Slavs.

[drran]

Read the docs.

https://www.mgh-bibliothek.de/dokumente/a/a011458.pdf