You unlock Black panther as a playable character and get this achievement by finding and picking up at least 5 of the black panther action figures laying around in the various acts (similar to the Scarlet Swashbuckler achievement for unlocking Daredevil). Some are hidden in odd places you may not naturally think to go, so go everywhere and look for the pop-up message that will appear as you pass over the figure. One example of an "odd place" is underneath the stairwell as you first enter the Iron Man lab in Act I's Stark Towers.
Here are locations for all of the figures. Once you get 5, the balance of figures will just add experience:
Beneath desk upon entering the 1st indoor segment.
Underneath 1st set of stairs in Ironman's lab.
Next to the pipe on the far wall of the Dive Room.
Behind a breakable Buddah statue at the very end of the map.
In front of the 3 large daises with green gas.
Area where you have to break crosses on one of the graves.
On the bow of the 1st Viking Longship on the right.
To the right of the fire pit at the end of the hall in the small area with a desk.
Treasure chest placed by the entrance to Asgard 3 & 4.
Behind the gate that you have to open with the spear.
Small secret area behind the 2nd power node.
Upper ledge where you destroy the large cylinder near the rocket.