High-Rise Invasion.

High-school girl Yuri suddenly finds herself on the rooftop of a high-rise building. She's trapped in a bizarre world surrounded by skyscrapers, where a masked man cracked open a man's head with an axe before her eyes. Finding a way to survive this bizarre world, find her beloved brother, and escape becomes her top priority, but she is beset by danger not just from the mysterious Masks, which possess both inhuman strength and cruelty, but other survivors turned cruel or desparate by the insanity of the high-rise world.