This Forge mod adds a creative-only item that prevents a player's health from reaching 0 as long as it's in any inventory slot, without the totem disappearing or losing durability.
Players still can take damage, but will always be left with half a heart.
As this is obviously an item that enables cheating, it is not obtainable in survival mode, and has to be given through the creative mode inventory or via command.
Why?
Because one person in my friend group wants to play relaxed and without risk while the rest of us want to play a challenging mod pack. This way, we can all play on the same server.
Additionally, since the holder of the item can still take damage, it enables the person to practice combat and learn to survive without the risk of actually dying.
So if you have a beginner or a younger sibling or child that you'd like to teach Minecraft to, this might be the mod for you! Just let them have the Totem of Immortality somewhere in their inventory and off you go!