Complex sentence: After Nerva commits suicide on the prospect of Caligula's rule, Tiberius, now dying, is at the mercy of being murdered by Caligula but Macro halts Caligula's action and commits the deed himself by choking Tiberius and thus hastening Caligula's ascent to the throne.
Split into simpler sentences: After Nerva commits suicide on the prospect of Caligula's rule, the now-dying Tiberius is at Caligula's mercy. However, Macro halts Caligula's action and commits the deed himself, choking Tiberius and thus hastening Caligula's ascent to the throne.