From the Publisher:

KeyForge: Mass Mutation was the most popular and hardest to find KeyForge set of the past. KeyForge: MORE Mutation is an ultra-rare set that pulls from the KeyForge: Mass Mutation card pool and includes new legacy cards from KeyForge sets published after KeyForge: Mass Mutation.

But there's more!

KeyForge: MORE Mutation will include additional Gigantic creatures. Specifically Logos, Saurian, and Untamed will each receive a new Gigantic creature, while Sanctum, Dis, Star Alliance, and Shadows will each receive two new Gigantic creatures. This means each House in KeyForge: MORE Mutation will have two Gigantics in their arsenal. Additional new cards that bolster Gigantics have also been added.