First, go to the mission Cortana, and right before you go down the halls to retrieve her, you should see some purple support beams leading to another area.

Get a plasma grenade and grenade jump up there, and go right until you can see the halls across the room.

Then there should be a little purple barrier, over the barrier is the skull.

The skull does not give you an achievement, it's been rumoured to be a second blind skull.

You can pick it up, melee people, so-forth. Enjoy!