From the Steam Store page:
Brave the Depths of a Forgotten Kingdom
Beneath the fading town of Dirtmouth sleeps an ancient, ruined kingdom. Many are drawn below the surface, searching for riches, or glory, or answers to old secrets.

Hollow Knight is a classically styled 2D action adventure across a vast interconnected world. Explore twisting caverns, ancient cities and deadly wastes; battle tainted creatures and befriend bizarre bugs; and solve ancient mysteries at the kingdom's heart.