Translation:Nobody knows what is going to happen.
That's... actually a very good question. Never thought about it. As far as I can tell, it attaches to "what" and "who[m]" when those are used in combination with a temporal aspect. So you get "inget vet vad som kommer att hända", "ingen vet vem som har kommit", etc.
But I would not place any large bets on it, and I would love to see if anybody else has any better explanations.
I found some Swedish grammar book that says that vad, vem, vilken etc. are followed by som when they are the subject of a relative clause, which seems to be the case here.
"When used as the subject of a relative clause, vad is followed by som." (Philip Holmes and Ian Hinchliff, Swedish: A Comprehensive Grammar, 188.8.131.52)
Where's the relative pronoun in the english translation? I'm expecting words like who or one.
In the English sentence, it's "what" here. But English might well use who too.
both 'vad' and 'som' are used when the relative pronoun acts as the subject in the relative clause.