Complex sentence: Under Activision Blizzard, the former companies of Vivendi Games turned into divisions of Activision with the exception of Blizzard Entertainment which was kept entirely separated from Activision and became instead a direct subsidiary of the holding company (like Activision).
Split into simpler sentences: Under Activision Blizzard, the former subsidiaries of Vivendi Games turned into divisions of Activision. The exception was Blizzard Entertainment who was kept entirely separated from Activision and became instead a direct subsidiary of the holding company Activision Blizzard (much like Activision).