The Gate of Death
"The Gate of Death (死門, Shimon), located at the heart. Releasing this gate uses up all of the body's energy. It makes the heart pump at maximum power and exceeds the power of every other gate. Opening this gate gives the user power that is even beyond the five Kage, but comes at the expense of the user's death shortly after."
Place your bets people, place your bets. Will we see Guy use this or not? I personally think that Guy dying wouldn't be a huge ordeal (all though I would be one sad, sad panda)
What will Guy be able to accomplish with it? Fend off Susanoo clones? Arm wrestle with a Perfect Suasanoo? Create an opening in Madara's defense so someone can seal him? (all though it's currently set up as Kakashi+Guy vs Tobito)