this happenened to me in Barbie Invasion. i played the marvellous mounted Sarmatians, horde settled in Greece, took nearly a hundred turns to sort out the debt and get to churn out Nobble Horsemen / bosphoran spears, beat the WRE force of 1st cohorts and auxilia palatina, take Ravenna, lovely "Sarmatian warlord " ancillary, bam! assassinated. next turn the heir assassinated. game over!

its the cool thing about the turks esp. in crusades, they seem to have a special affinity with assassins. "he should Not have offended you, Sultan."