Cards that replace themselves are usually pretty good. But the best part about Relentless Dead is that you can actually get back another Relentless Dead from your graveyard with this second ability. Best of all, since both of those "when it dies" abilities trigger at the same time, you can actually use both. Returning it to your hand and getting a Zombie onto the field from the grave is extremely powerful. This seems like an easy four-of in a competitive Zombie deck. Zombies were good during the first Innistrad block, and perhaps they will be good once again.
It’s also cool that Relentless Dead’s art references that of a very popular enchantment called Endless Ranks of the Dead. Endless Ranks is extremely popular in Zombie decks in both the casual environment and in EDH / Commander. Relentless Dead seems like an auto-include in any Zombie EDH deck, and it's popped up in plenty of Gisa and Geralf, The Scarab God, and Grimgrin, Corpse-Born decks.
Relentless Dead would shine in Standard, especially in Mono Black Zombies and Mono Black Aggro. The two-drop Zombie has also popped up in some Modern Mono Black Devotion decks alongside Bloodghast, Geralf's Messenger, and Phyrexian Oblterator. With future support, may even make Zombie Tribal in Modern a reality.