Complex sentence: He is best known for publishing the first Encyclopedia of the Bahraini History since 3000 B.C. until the modern Bahraini state; the hugest and first of its kind in the GCC region.
Split into simpler sentences: He is best known for having published the first Encyclopedia of Bahraini History, stretching from 3000 B.C. to the modern Bahraini state. It is the largest and first work of its kind in the GCC region.