Where does “Zarentum” come from?

Zarentum (German) comes from German Zar, from Russian царь, from Old East Slavic цьсарь, from Old East Slavic цѣсарь, from Proto-Slavic cěsařь, from Proto-Germanic kaisaraz, from Latin Caesar, from Latin caesaries — hair.

Zarentum (German): tsardom territory ruled by a tsar