Complex sentence: Tacitus wrote a speech which he attributed to Calgacus, that Calgacus gave in advance of the Battle of Mons Graupius, in which he describes the exploitation of Britain by Rome and rouses his troops to fight.
Split into simpler sentences: Tacitus wrote a speech which he attributed to Calgacus, saying that Calgacus gave it in advance of the Battle of Mons Graupius. The speech describes the exploitation of Britain by Rome and rouses his troops to fight.