As it is currently, without a comma after universe, this sentence is saying that the shack turns out to be a door while the hooligans are about to find out. *shakes head* Add the comma.
An abandoned shack turns out to be a door into another universe, as a band of local hooligans is about to find out.
Sometimes I really don’t like these writing prompt things I use for most of my “glitch” posts. The verb tenses (people who create the prompts seem overly fond of present tense but unable to keep it straight) are often awkward and unclear… Try past tense instead: An abandoned shack turned out to be a door into another universe, as a band of local hooligans found out.