The Rogue God Series: The Coming Darkness

A Dark Elf (the last survivor of his peers) attempts to fulfil his lifelong ambition to remove the false gods from the land by performing a magical nisus, opening a portal to return them to their old world. While casting his spell, the dark elf loses control and inadvertently creates a doorway to the wrong world.
Shocked at discovering the impossible, two outlanders travel through the portal. On their arrival, they see vast riches and dream of great wealth. They learn that this land is on the verge of turmoil as Gods and mortal races scheme to enforce their will.
Cerridwen, a goddess who has been imprisoned by the greater gods for centuries, unleashes her plan to regain freedom. Using the darkest of magic to look into the future, she discovers that the two outlanders and a coven of Wicca witches are destined to stop her at the height of her power. In alarm Cerridwen orders her son Afagddu to murder the witches to change the future - only to begin a chain of events that will have disastrous effects on all of the peoples of the land.
The outlanders, overwhelmed by the turmoil which has confronted them, attempt to leave, only to learn they are cut off by an army of orcs. Despite narrowly escaping the orcs and other incomprehensible horrors, the two outlanders learn they are part of a prophecy that must be fulfilled if ever they are to return home.