Return of the Living Dead

At a medical supply warehouse in Louisville, a foreman named Frank tries to impress the company's newest employee Freddy by showing him military drums that accidentally wound up in the basement of the building; the drum contains the remains of an military experiment gone wrong that inspired the film Night of the Living Dead. Frank accidentally unleashes the toxic gas, which renders the two unconscious. When waking up, Frank and Freddy find that the gas reanimated a cadaver inside a meat locker, and contacts their boss and warehouse owner Burt Wilson to help them deal with the situation. When decapitation cannot kill the cadaver, Burt decides to bring the corpse to the nearby mortuary to have its dismembered parts burned in a gambit to destroy it once and for all.
Meanwhile, Freddy's friends learn of his new job from his girlfriend Tina. The group, consisting of Spider, Trash, Scuz, Suicide, Casey, and Chuck, decide to pick Freddy up after he finishes his shift and wait inside the local cemetery. Tina leaves the graveyard and enters the warehouse to look for Freddy. Her search leads her to the basement, where she unwittingly draws attention from the half-melted corpse from the drum barrel (dubbed "Tarman") who was initially thought to have dissolved. She locks herself inside a metal closet after a failed attempt to run, but the zombie finds a chained hook and tries to pull the door off.
At the mortuary, Burt gets the mortician Ernie to burn the remains, not realizing that it causes the gas to contaminate the air and bringing a toxic rainfall to resurrect the dead in the graveyard. The acid rain forces Freddy's friends to refuge inside the warehouse. Once inside, the group hears the basement commotion and immediately rush down to rescue Tina, but Suicide is bitten by Tarman in the process. This forces the group, including Tina, to abandon Suicide and barricade the door to stop Tarman from coming up. Outside, the group runs through the cemetery to look for Freddy, but are split up when the zombies begin rising from their graves. While the group is running Trash gets surrounded by and eaten by zombies, ironically dying the way she described earlier as the worst way to die. Tina, Spider, and Scuz go to the mortuary, Chuck and Casey head back to the warehouse where Tarman remains trapped downstairs.
Frank and Freddy have grown increasingly ill from their exposure to the gas and a medical test from paramedics implies that they are no longer alive. When Burt and Ernie find out about the graveyard zombies, they quickly barricade the mortuary after a failed attempt to escape using the ambulance. Scuz is killed while protecting the barricade and the zombies continue to eat the paramedics and police who arrive in the area. With Frank and Freddy showing signs of becoming zombies themselves, Burt has them locked in the chapel and Tina stays with Freddy. Meanwhile, Trash rises as a zombie and starts a semi-organized horde that attacks the reinforcements head-on.
Freddy succumbs as zombie and tries to eat Tina, but is blinded with acid thrown by Ernie. Frank also escapes and cremates himself inside the retort. Burt and Spider use a police car outside to escape, but the growing horde of zombies leaves them unable to come back for Ernie and Tina. They retreat to the warehouse and the car explodes due to a gasoline leak. Burt and Spider reunite with Casey and Chuck, and go to the basement's phone after Burt knocks off Tarman's head with a baseball bat. With the police massacred by the zombies, Burt decides to call the military drum's number. The call goes to Colonel Glover, a military officer looking for the barrels. When Glover learns that the gas has been released, he activates a containment protocol with a nuclear artillery shell to destroy the area. Just as a blinded Freddy breaks into an attic where Ernie and Tina are hiding, the shell destroys 20 square blocks of Louisville and ultimately kills both the survivors and zombies. With footage shown of the destroyed area, Glover describes the outcome with less than 4000 dead and dismisses reports of the acid rain, which soon restarts the zombie rising all over again.

Question: Besides the zombies, what forces the group inside?
Answer: Acid rain