Okay, so. The Ghost River Triangle is basically a place that is cursed land. The Revenants (ressurected outlaws killed by Wyatt Earp) are trapped inside of it. If they go outside of it, they basically burn alive. I mean, they can't die until Peacemaker (aka Wynonna's gun that used to be Wyatt Earp's gun) shoots them. So it's basically just very very long and drawn out burning alive with no end in sight.
The Stone Witch is actually Constance Clootie. She was married to this demon and when Wyatt killed him, she cursed the Earp family. The Earp heir turns 27, has to kill all the Revenants. If they die, everyone ressurects again.
Keeper of the bones I guess is about her dead sons she was trying to bring back to life or something. I had one skull. My Uncle died, so I guess he didn't get the chance to fill me in fully on that whole situation.