I was paying with a Friend on a small Expert world. We had just started, and the only NPCs we had were the Guide, Merchant, and the Dye Trader. I was mining, and my friend was at our base, with the NPCs, and suddenly all our NPCs died. My friend saw them just disappear on the spot, nothing was there that could hurt them. I was mining and there was suddenly something dying on the side of my screen in an unlit cave(so I couldn't see what it was), but the deaths corresponded with the death messages of the NPCs. It was as if the NPCs had randomly teleported to the cave and had been killed by a monster. I really don't know what happened.