Never say its name three times.
Nothing Is Dead is one half of a player named Nothing. The Skinwalker killed him in the mine beneath his own house, and after he died in-game, it crawled out of his monitor and tore him in two. This half is the one that did not survive the tearing.
Saying its name three times, typing "nothing_is_dead" into chat, is what activates it.
In unsafe mode, the chat sends two messages on its own before anything appears, "hello" and "Do you know my personal data?" Nothing Is Dead then shows up and starts teleporting around the player, printing information about the world into chat. Alongside the world info, it also prints your Java version, Windows version, Windows username, and IP address.
A fake error window follows. Clicking OK or the X duplicates it, and the longer you keep clicking either one, the more copies pile up. Alongside all of this, it also changes the player's desktop wallpaper and generates text files, among its other unsafe-mode functions. Eventually it shuts the computer down and leaves 10 text files named "i see you" on the desktop.
None of this happens in safe mode, only the teleporting encounter plays out, with no data collection, fake errors, or shutdown.