If something is necrosed, it is dead or dying, especially body tissue.
The word necrosed is used in medicine to talk about something very serious and sad.
Examples:
The necrosed tissue had to be removed.
The doctor said the wound was necrosed.
The necrosed part of the plant turned black.
The necrosed tree looked very sad in the garden.
The necrosed tissue was as hard as a rock.
The necrosed part of the body was very smelly.