The Raid Journal and Datamining indicate that Zovaal wants to blow up all the Zereths except Zereth Mortis.
For Death to be dominant Zereth Vitalis, Zereth Ordos, Zereth Tumultus, Zereth Umbra and Zereth Lumen have to go.
Zovaal wants only Zereth Mortis to exist and to personally run Reality with it.
Incidentally the first Zereth he is targeting according to the Raid is Zereth Ordos AKA the Titan Azeroth and he is doing it during his own Raid Encounter.