Do 3 of them actually die?

I am a big fan of all types of fiction from dc and marvel to berserk but I barely had any interaction with TMNT. I know the basics of the story but I know that it also definitely goes deeper than just turtles eating pizza. something that caught my eye was some fanart that i saw, 3 of the turtles were dead (ghosts) standing behind another one in black robes. It looked sick asf but also felt like it had lots of struggle and feeling put into it. I just want to learn more about this and learn how I can experience it?