Enduring Tenacity is a strict upgrade for the classic Sanguine Bond, having more staying power and coming out for cheaper while providing a damage-based win-con for gaining life
Adding more value to him is the fact that late in the game you can transform him and either dig for more resources, blow up any creature/planeswalker or recur something from any graveyard
You get green-blue's Town payoffs alongside black's removal and value cards to shore up any weaknesses
I like to think about a Marina Vendrell EDH deck as the Niv-Mizzet Reborn for enchantments
While she costs only three mana one red, one green, and one of any color because her saga sides final effect generates 10 mana spread evenly across all five of Magics colors, her color identity is also all five
Having more ways to get the mana you need when you need it streamlines every other part of the game, after all