The Wicker-Man is a landmark from the Witchery mod. These appear as human-looking structures built out of Hay Bales. These typically have a monster spawner underneath them, usually a zombie or skeleton. Wicker-Men can be found basically everywhere. Lighting a Wicker-Man on fire may attract a Horned Huntsman. As of version 0.19.0, a Mystic Branch using the Incendio effect can ignite the Wicker-Man.
The kinds of spawners found underneath Wicker-Men can be configured in Witchery's configuration file.