Risen Executioner is a 4/3 Zombie Warrior for 2BB (2 colorless, 2 Black) that can't block. He gives other Zombie creatures you control +1/+1. What makes him different from the other Zombie lords is that you can cast him from your graveyard. The drawback is that you have to pay 1 colorless mana more to cast him for each other creature card in your graveyard. If he's the only creature in your graveyard, though, you're in luck. Basically, he's a more expensive Gravecrawler that pumps other Zombies.
Since even before the set released, Risen Executioner has been a bulk mythic rare. There was some early interest in him for inclusion in Zombie Commander decks such as Ghoulcaller Gisa and Grimgrin, Corpse-Born. He's a bit redundant even in those decks. The Executioner isn't a bad card. It's just one of those mythic rares that is just a bit too good to be a rare, but not really strong enough to hold much value as a mythic. In Limited, I can see why this was a bit too strong to be a rare. It could do some serious damage.
Zombies aren't really big in Constructed right now. Even then, there are just better Zombie lords than he, Risen Executioner is a nice Zombie to have that can recast himself. The boost effect is only going to be relevant in Zombie Tribal decks. Even still, there are better choices like Death Baron and Cemetery Reaper that do the job at 3 mana, anyway. Recasting the Executioner in the later game simply doesn't seem worth the mana, not in a deck archetype that depends on having creatures in the graveyard. He's basically a Commander-only playable.