Impaling a head requires you to get 25 fatalities on a specific character, and have progressed to the entrance to Goro's Lair in the Krypt. I won't explain how to get there, because it's pretty straightfoward, and the game kinda holds your hand.
The easiest way to boost the 25 fatalities is to go to klassic towers and scroll over to the endless tower. Make sure the first fight is a non-DLC character, as the DLC characters don't drop heads. Any character other than Shang Tsung, Nightwolf, Joker, Spawn, Robocop, Sheeva, Rambo, Terminator, Sindel, Mileena, Fujin, or Rain should work just fine.
Enter the tower with the intended first fighter. Beat them and do a fatality. Afterwards, load in the second fight. After it starts, pause and return to tower select. You should be able to repeat the tower with the same first fighter. Do this 25 times and you'll get the head in a notification after the fight.
Once you have the head, go to the entrence to Goro's Lair in the Krypt, and approach the spike that has the bust of the head of the character you just killed 25 times, and place it down. You should get the trophy, and a bunch of gear for the character.