Director and co-showrunner Miguel Sapochnik is leaving HotD for season two. He has a deal to do other projects for HBO and is apparently exhausted after spending 3 years getting this show off the ground.
Sapochnik grew to fame in Hollywood for directing some of the most intense episodes of Game of Thrones including Hardhome (the episode where John Snow and the wildlings are driven off by a massive army of the undead**), Battle of the Bastards (for which he won an Emmy award), and The Long Night (for which he was nominated for an Emmy). Basically, the most memorable battles in the series were directed by him. He also directed the first episode of HotD.
This will leave co-showrunner Ryan Condal as the sole dude in charge starting in season two. Condal is primarily known as a writer (he wrote the first two episodes of HotD). It is hard for an outsider to say how much of season one is really being steered by Sapochnik and how much by Condal. I know some folks are freaking out a bit at Sapochnik leaving the show because he is so beloved for some of his GOT directing but I'm not gonna freak out yet. Remember, unlike GOT, this show has a pretty well-mapped out story from beginning to end because GRRM has already finished this book. I'm not sure how long HBO is planning to stretch this sucker out (I would suspect 3 or 4 seasons, at least) but at least there is a road map to guide the writers and showrunners, whoever they may be.
-Jason "**-the moment in Hardhome where the Night King raises his arms and all his soldiers, plus the dead wildlings, come back to life is still one of the great moments in TV history" Evans