Complex sentence: Pliny the Elder is the earliest writer to mention Adulis (N.H.6.34), who misunderstood the name of the place, and thought its name meant that it had been founded by escaped Egyptian slaves.
Split into simpler sentences: Pliny the Elder is the earliest writer to mention Adulis (N.H.6.34). He misunderstood the name of the place, thinking the toponym meant that it had been founded by escaped Egyptian slaves.