Would or will. A question is about a certain future event

"Ohad knew he had a few hours before they would open a wormhole to Mumbo Booboo."

The sentence describes events taking place in the past. The "would" acts as the simple past of will here.

https://www.dictionary.com/browse/would?s=t

As @Phil Sweet commented, "Ohad knew he had a few hours before they opened a wormhole to Mumbo Booboo" would be more common.

Ohad knew he had a few hours before they open a wormhole to Mumbo Booboo

That's English subjunctive mood as in "I ask you leave now".