This variant has been tested by me and my SO. It makes the game harder, but it is still doable, even with the worst monster deck. It also maintains some surprise on the appearance of Shadowrifts, but avoids the situation where they can come up right off the bat and make the win feel cheap.
VARIANT
Preparation: Take half of the Shadowrift(s) needed (rounding down), and shuffle it/them into the monster deck as normal. Then divide the deck in halves. Shuffle the remaining Shadowrift(s) into what will be the bottom half of the deck.
Game Play: During the add-power phase, add power equal to the number of players plus 1.
VARIANT
Preparation: Take half of the Shadowrift(s) needed (rounding down), and shuffle it/them into the monster deck as normal. Then divide the deck in halves. Shuffle the remaining Shadowrift(s) into what will be the bottom half of the deck.
Game Play: During the add-power phase, add power equal to the number of players plus 1.