This week, the developers at 343 Industries preview the concepts behind, and the work that went into, creating the newest Halo baddies. Dubbed the Prometheans, they will use their bionic abilities to put the hurt on Master Chief. One interesting tidbit revealed in the documentary short below is that the different Promethean enemy types will work together based on their abilities, making Halo 4 combat more tactical than past Halo games. The developers state that the "knight" Promethean starts out more powerful than a Covenant Elite.
Also, though the redesigned UNSC weapons were shown off in a trailer one month ago, the futuristic Forerunner weapons are detailed by developers in the latest documentary short, seen below.