[@CollectorOfMyst] The Darkness is at 1/15, when it reaches 16, it has escaped the mine.

Every monster that escapes the mine moves the darkness forward two steps, every large monster (like the night terror) move it 3 steps.

At 3/15, 6/15, 9/15 and 12/15 a Darkness Card would trigger, but in this specific mission, all monsters move forward two squares when this happens instead of a darkness card.

With the Void Spiders and Stranglers currently near the exit it'll be difficult to pull it back, but it's still possible.

Edit: For every 3 creatures killed or every 1 large monster, it gets easier for the heroes to break the assault (aka the win condition).