An Aether boss. Came across this kind twice, not entirely sure if there is more than one kind. It will appear as a giant, inactive block at first. Hit it with an axe to activate, and harm it (Diamond, or Aether grade equivalent material preferably. Even with those, it takes a LONG time, dunno about anything lower) Two stages by the way. The blue eyes here, and red eyes. Red eyes might be stronger. Not quicker though.