So he was a deeply religious person. Spoke about paradise and he finally accumulated enough slaves for his afterlife. He stopped killing people because he met his goal. What a sick bastard.
I saw a little documentary on that as well. Not sure if/when that'll be cracked.
I was talking about Cicada 3301 - I spent almost two hours going back through Youtube history to finally find it. It seems inconclusive whether it was actually "solved" or not.