The abode of the dead; hell.
Old King Dave is passing the throne on to his son, Solomon. But he has some unfinished business. This bloke named Joab has been causing a ruckus, killing some people, “avenging in time of peace for blood that had been shed in war”.
Dave doesn't like this.
So Dave gives young Solomon some instructions regarding Joab. And it turns out Joab isn't the only one that old Dave has a gripe with. A chap named Shimei also gets a mention in the last will: “Bring his grey head down to Sheol with blood.”
As soon as Dave kicks it, Solomon (proving that he is just as much of a badass as his father was) goes straight out and—takes care of business.