In writing a scary story based on The Gettysburg Address, there are several things that are necessary.
First, make sure you are aware of all the details related to the speech that Lincoln delivered to those at the site of the battle at Gettysburg. Research the reason Lincoln traveled to Gettysburg to conduct...
...a dedication ceremony that transformed the site of the Battle of Gettysburg into a national war cemetery.
Having as many facts as possible helps lend your story credibility. Setting the mood is important, as well as using devices such as foreshadowing, suspense, inference, etc.
The mood is how the reader feels when reading a story. Foreshadowing are hints at what is to come in the future. Suspense is a sense of uncertainty, defined as...
Suspense in fiction results primarily from two factors: the reader's identification with and concern for the welfare of a convincing and sympathetic character, and an anticipation of violence.
This example is from "The Name of the Game" by Elizabeth Spencer:
He was an innocent, this boy; the other boys were out to get him.
Describing a scene or introducing a threatening situation can set the mood. In Poe's "The Cask of Amontillado," the first line promises violence:
The thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge.
Suspense creates a sense of anxiety—perhaps even fear. Inference is a device that offers clues without telling the reader what to think: the reader makes his or her own decisions. The most significant uses of these elements provide a story that leaves the reader on edge throughout the reading. Providing unexpected details at the end can make a story even more than scary: it can be horrific.
In Faulkner's "A Rose for Emily," Emily Grierson comes from an old family of the Deep South. Early in her life, she shows indications of insanity. The author switches the sequence of events, writing some parts in chronological order, switching back in time (with flashback), and returning to the present again. In this way the reader is unable to follow the unusual behaviors Emily exhibits. She grows old (with iron-gray hair) and reclusive. When she dies, the townspeople enter her home. Not only do they find the body of her murdered lover, but discover that she has slept next to his body for many years, even into her final years, but including the detail of her hair on the pillow: not only is this a surprise, a horrific development, the discovery comes to the reader through drawing an inference.
Then we noticed that in the second pillow was the indentation of a head. One of us lifted something from it, and leaning forward, that faint and invisible dust dry and acrid in the nostrils, we saw a long strand of iron-gray hair.
In writing a scary tale regarding The Gettysburg Address, I would suggest making use of these literary devices. I would use sensory details to make the writing more realistic (note the "iron-gray hair"). You can develop your plot to reflect something that happened at the dedication ceremony, you can tie the two time periods together with a ghost from the past meeting a teenager from the present, or you can turn it into a mystery where the teen discovers something that will discredit someone who was honored at the site, perhaps on the memorial, and his family will do anything to keep it a secret. These elements should provide you with the ability to write your scary story.