A supply truck rides up to the prison. It stops at a security check at the front gate. 
The guard holds up a clipboard. Strange, there were no deliveries scheduled for today. The guard leans out of the window in the booth. "Pardon me, but what are-- GASP!" 
Two long dark tendrils snap from the driver's seat, striking the guard down. A small bit of black residue begins growing on the guard. A tendril reaches for the gate controls and flips a switch. The gates open, and the track enters. 

Inside, the inmates are being taken back to their cells. All is normal, until a guard rushes in. "Warden Noble! Somebody crashed a truck into the main power housing!" 
The lights go out. The inmates are hopeful; is this a jail brake? Are try being liberated? 
A man's scream echoes through the corridor. Crashing and wrestling. A shrill scream, inhuman, followed by deafening roars. 
No, not a jail brake. The inmates are as frightened as the guards. Something has gotten into the prison. 
In the darkness, the inmates watch and wait. Suddenly, several dark figures rush into the large detainment area. They move like animals, yet look like men. They pounce on the guards, who scream in horror. Slowly, the guard's screams are gurgled, a black ooze crawling over them, dripping from the creature that attacked them. 
After the guards are defeated, the creatures begin ripping the cell doors off their hinges. The inmates cry out, as they are taken by the creatures. 
Meanwhile, in a cell across the room on then ground floor, a lone inmate watches in horror. He sees the dark figures attacking the prison's inhabitants. Are they eating them? 
There is movement at the entrance to the corridor. The inmate turns and sees another creature, except this one is different. It walks like a man, with a powerful, ominous stride. 
It looks at the inmate. 
The inmate's eyes widen, and he scrambles to the back of his cell and crumples up into a fetal position. The new creature calmly walks towards the cell door. 
The creature grabs the bars and rips the door off it's hinges. It walks over to the inmate and grabs him by his shirt collar. The inmate is hyperventilating. 
"Do not be afraid." The creature's voice is smooth, masculine, soothing, even. "I will not harm you." 
The inmate struggles to speak. "Are... Are you g-going to... To free us?" 
"No. I am not." 
Darkness grows on the inmate's shirt. It grows upward, covering his head, and downward, covering his body. When he is fully consumed in the blackness, the creature drops him. 
The inmate is no more. Now, another twisted evil being slowly climbs to it's feet. It's legs wobble, and so it falls to it's knees. It hops past the creature and out into the open. It lets loose a terrible scream, then lumbers away to find a new victim. 
Later that night, all is quiet again. A convoy of the prison's vehicles drive away from Kryptarium Prison. 
Inside the prison, everybody is gone.