This side mission is very simple, and likewise, has limited rewards to reflect this. Mostly, the mission is designed to teach unwary players that it is not good to step on every HEX they come across, at least not without warning or coordinating with allies first.
To activate the mission, merely clear away the Rocks blocking the HEX towards the center of the map, along the middle bridge. Standing on the hex will trigger an audio cue, causing the Fearsome Brain Bug to awaken. It will unburrow (not necessarily from underneath the hex), and then proceed to chase and attempt to kill the nearest threats.
If alone or unable to deal with it, it can outrun and severely wound or kill almost any player. Multiple players must work together to tank, stun, disable, and kite the creature. Using weapons effective against Massive Enemies and offensive skills is the best way to damage it. This mission is present in all campaigns, and can be activated at any time. Killing it rewards 1 Karma point to each player.