Complex sentence: Sviatoslav's appearance has been described very clearly by Leo the Deacon, who himself attended the meeting of Sviatoslav with John I Tzimiskes, to be a blue-eyed caucasian male of average height but of stalwart build, much more sturdy than Tzimiskes.
Split into simpler sentences: Sviatoslav's appearance has been described very clearly by Leo the Deacon, who himself attended the meeting of Sviatoslav with John I Tzimiskes. Following Deacon's memories, Sviatoslav was a blue-eyed caucasian male of average height but of stalwart build, much more sturdy than Tzimiskes.