Complex sentence: Whereas other histories merely describe Foveaux as some able and efficient administrator who became Lieutenant-Governor at Norfolk Island, Holt saw him, and graphically described him as the greatest tyrant that he (Holt) had ever known.
Split into simpler sentences: Whereas other histories merely describe Foveaux as some able and efficient administrator who became Lieutenant-Governor at Norfolk Island, Holt saw him far less blandly than that. Holt graphically described Foveaux as the greatest tyrant that he (Holt) had ever known.