Greetings Captain Kirk! tvbythenumbers is reporting that "William Shatner (Star Trek) is set to guest star in a four-episode arc on the Syfy fan-favorite series in 2015. Shatner will play a pivotal character that has the potential to forever impact the fate of the town of and its troubled residents."
tvline.com puts it this way:
William Shatner will seemingly throw the Prime Directive out the window when he sets foot in Haven later this season. The iconic Star Trek captain recently teased on Twitter his gig on the Syfy series. Now, TVLine can exclusively reveal that Shatner will recur in a four-episode Season 5 arc in “a seminal role,” as a pivotal character that has the potential to forever impact the fate of the town of Haven and its Troubled residents.