Complex sentence: Alexandros Ypsilantis was contacted and asked to assume leadership of Filiki Eteria, which he did in April 1820 and began active preparations for a revolt, as well as for setting up a military unit towards that purpose named as the Sacred Band.
Split into simpler sentences: Alexandros Ypsilantis was contacted and asked to assume leadership of Filiki Eteria, which he did in April 1820. He began active preparations for a revolt and with the setting up of a military unit for the purpose that he named the Sacred Band.