Complex sentence: It is a unique place in the Eastern Europe, where are located the original paintings of Mark Rothko--a great artist of the XX cent., a founder of the abstract expressionism movement, best known for his enigmatic color field paintings.
Split into simpler sentences: It is a unique place in the Eastern Europe, where the original paintings of Mark Rothko are located. He was one of the great artists of the 20th century, a founder of the abstract expressionism movement, best known for his enigmatic color field paintings.