Ancient "dead gods" from the Shadowfell had influence over Barovia; their essences are now held within the Amber Temple, contained their by ancient wizards.
Despite being trapped in amber sarcophagi, they are still alive and able to communicate with anyone who makes contact with them.
It is believed that Strahd von Zarovich bargained with these Dark Powers in order to gain the power to rule over Barovia but the land was then pulled into the Shadowfell and became a demi-plane.