its a mode unlocked after completing SoS mode whereby all enemies die after a single hit. You die after 3 hits (you have 3 gold orbs to use at the level start).
For instance, a usually challenging part in a lift involving a blitz, x3 mephisto 's and two frosts (this bit was really hard on SoS mode) can be cleared within a few seconds (press X a few times).
I was expecting it to get tougher as i went on, but it easy all the way through
Its seems a very weird mode as it is above SoS and DmD modes. Hell or Hell mode looks VERY challenging. Apparently you can get hit 3 times, but enemies now have SoS health/difficulty.
Also levelling up my character has a finally annoyed me. I had racked up 76,000 proud souls and went to buy the last 2 abilities for Nero. One cost 55,000 souls and the last around 60,000. After buying one of these skills the last skill now costs 101,000 proud souls!!!! That's a few play through minimum. totally ridiculous. There's having challenging achievements, then there are achievements that blatently take the pea. I'm annoyed as i know i'll re-play through for completeness sake - I'm a sucker for type of stuff unfortunately
On the bright side, Heaven or Hell mode makes getting the "Legendary Devil Hunter" (10,000 kills) a little easier.
I think next i'll try DMD mode to see exactly how challenging this actually is (I imagine very challenging, although you do get a load of proud souls after completing each level).
EDIT: at least in DMD mode you get a ton of proud souls for your efforts. Just finished mission 2 with an A rank and 5700 proud souls

Although the blitz just owned me in mission 3
