To create such an item you have to work with battle events:
-First of all you'll need to set up a State which can't get removed and gives resistance to death and a
common event with the trigger set to none, which turns a switch on.
-Now set up the Item itself: It has to
trigger your common event and inflict the invincibility state. You can do that in the effect panel. Set up the rest as you wish.
-Last thing you need to do is setting up the actual battle event in the troop tab. I guess a screenshot isn't much help as I have the german version but to give you an impression of how it should look like, here it is:
So the conditions of this event are that Eric has 0% or less hp
and the set up switch is turned on. The Due to the invincibility state he won't die so the battle isn't lost. The switch is there to check if it is actually the Item that makes him invincible and not something else.
The first command is the recover all command, to heal Eric. You can also use change hp if you don't want full hp.
The script call after that shows animation 42 on actor 1. As there is no visible feedback on actors by default, this will do nothing unless you have a script that does animations on actors. (like Yanfly battle engine or any actor battler script)
After that the event removes the invincibility state
and eventually turns the switch off again.
Note that you have to copy this event to every single troop you use in the game.
EDIT: Remove the switch and just put the battle event inside a conditional branch, which checks if the state is applied. (don't know why I didn't come up with that in the first place) Also I forgot to mention that the span should be round. (It is in fact battle in the screenshot...)