Putting aside the cultural meaning of “Zooku”, I’d probably prefer to match it to something like “dungeon break”, “monster parade”, or something edgier like “emergence event”. Though in the context of a transmigration from a game world into reality, calling a horde of monsters a “zerg” has some interesting implications about no creatures being an NPC. Not that this manga would be subtle enough to suggest that monsters might be anything but.
Anyhow, if someone can find a half literal translation of “Zooku” I’d go for that. Maybe it’s a strange Japanese portmanteau/acronym, or reference to folklore or an old game. If it’s just a made up word then keep it as is, change it for something more meaningful, or even just make up a completely different word. Doesn’t really matter. Zork isn’t terrible.