When stepped on, it takes half a heart of heath and will knock the player back several blocks. It isn't dangerous when it is just a single block, but if it is many, it could trap the player in a corner, or just knock them off a high surface. They were added in the 7.2 'Ancient Kings' Build.
Like most other things in the mod, it does not spawn naturally in the world.
A singular block looks like four obsidian blocks wittled down to rough spikes. These spikes are long and sharp, as well as purple and black, like the normal obsidian block. They are relatively small, as they don't even reach the players knee.
They are normally used by Anu to slow down and kill the player when he is weak, but they can also be crafted, albeit with a somewhat expensive recipe. They can also be mined when Anu spawns them, although you'll need a diamond pickaxe to mine them.
When a sufficient amount of them is gathered, they can be used to guard certain areas from Mobs or other players, look at the picture on the right for an example. They can also be used for decoration, though this might be somewhat dangerous.