From Brotato Wiki
Weird Ghost is an in-game item that can be bought from Shop or can be found in a Crate. Most Items increase the Stats of the player while some of them provide unique effects.
Weird Ghost provides Max HP at a discounted rate, but you start the next wave with just 1 hp. It is a gamble. Be careful with taking Weird Ghost if you are playing Sick or have the item Blood Donation or Bloody Hand.