In 2013, he began to shift his focus away from animation and documentary works and towards horror short films. According to Sandberg, his original goal with filmmaking had been to make horror and sci-fi films, and he felt he needed to have some existing work in that field in order to get funding for future work. Lacking any funding of his own, his shorts were made with no budget—his equipment and props consisted mostly of items he already had or could buy very cheaply, and his only cast and crew were himself and Lotta Losten, his wife. He released his films on both Vimeo and YouTube, using his alias ponysmasher.
In 2013, he released his second horror short, Lights Out, which centered around a monster that only exists when the lights are turned out. The film was a viral success, and quickly garnered millions of views. According to Sandberg, his life changed "in every way imaginable" after Lights Out; he began receiving tons of messages from film industry people who wanted to work with him.
Sandberg married actress, designer, and photographer Lotta Losten in 2013. She has featured in many of his short films, including Lights Out.