From the Super Mario Wiki, the Mario encyclopedia
Lethal Hammers are a rare variety of Hammers appearing in ScareScraper mode in Luigi's Mansion 3, added in version 1.3.1. They are red and black, glowing Hammers that will empty every one of the players' HP if hit. They have 350 HP, almost double that of a regular Hammer. They are defeated the same way as normal Hammers.