Adam and Fromund completed one part of their plan — they dealt with the volcano beasts, but immediately made a mistake and allowed one obsidian golem to consume the core and evolve.
Did this mean that they would now be unable to find the Twilight Crystals, and their mission would be a failure?
Of course not.
The Kirani Bugs continued to fly in from the sides, growing in number because, unlike the volcano beasts, the Obsidian Guard could not destroy them at a great distance. He had become much stronger, but he was alone.
However, the Kirani Bugs were in no hurry to attack. Most of the core was in the Obsidian Guard's chest. If they killed him soon, they would be able to obtain the core almost in its entirety.
It was a risk the Obsidian Guard had to take to evolve in such a short time.