The Trap by H. P. Lovecraft and Henry S. Whitehead

Two schoolboys find an old mirror in an abandoned house, but one of them vanishes after staring into it too long. The other seeks help from a priest, and they soon realize the mirror is a portal to a strange and terrifying dimension. The story builds on eerie imagery and growing panic. The plot escalates toward a race against time to rescue the lost boy. A classic mirror horror tale with cosmic consequences.