In WoW certain bosses had attacks literally called 'Void Zone' but it was not the general term used for harmful AoE effects. Aside from that example, when I think of 'void zone' I think of the area underneath the world that is often reachable in many MMOs due to slightly incomplete maps/exploiting.

Anyway, the TC is talking about harmful AoE effects by bosses, you can think of the term 'void zone' simply as 'the place you don't want to be standing during the fight'.

The most universal term for this, while being less generic than 'AoE', is simply: The Fire. Don't stand in the fire!