- The brothers learn of the massacre of their sons & allies, and summon Draupadi to break the news. As usual, all she wants is revenge. Thus the Pandavas go off on one final mission to avenge the death of their sons. They track down Ashwatthaman and find him hanging out with Vyasa and Narada. As soon as they set eyes on each other, the boy Brahmin sets off his Brahmashira weapon, which is 100s of times more powerful than the Brahmastra, and sounds a lot like a hydrogen bomb-- "All living things are destroyed and no rains fall for 12 years."
The Rishis step in and order Arjun to put his guns away, but Ashwatthaman cannot retract this weapon once it is unleashed, so instead, he turns it loose on the wombs of the Pandava women, making them sterile and killing their one remaining heir-- Uttara's unborn son Parikshit.
Fortunately, Krishna revives the kid, but there shall be no more Pandava offspring after this.
To help explain how Ashwatthaman was able to pull off this massacre single-handedly, Krishna tells a few stories about Shiva, and just how temperamental and dangerous he can be. In fact, Shiva really comes off as more of a force of nature than a Saddhu living in the hills.
My guess is, that if we understood the physics of Time, we might realize that Shiva stands in for one of the main functions of time, which is change. The other aspects of time are novelty (Brahma), and the moment of Now, which I believe is represented by Vishnu.