You're probably here from the Dawn of the Void Modpack. THIS IS NOT FROM THE EREBUS MOD. IT IS FROM VoidDecay This block consumes and replaces other block as a non-entity and non-block with no hitbox and is partially transparent with a purple tinge to it. This block is caused by the Erebus Keystone and may spawn naturally in the Erebus dimension. Player and mobs take damage when inside of it. It also destroys items and cannot be replaced by another block. This block consumes every block and even consumes bedrock. As far as noticed by the writer of this it only travels if there is a block it can travel to adjacent to it. May despawn if it cannot move in any direction. There are a lot of unknowns about this block but if the player builds a skybase they may escape the corruption if no blocks are adjacent to any other blocks. You can destroy it with Cleanse TNT. Very similar to the taint in Thaumcraft mod.
2016-07-20 18.18.25

Inside Of it in Creative

2016-07-20 18.18.32

What the player first sees of the corruption. Beware!!!