Not to neccessarily challenge Blizzard, she/he is only partially right. I shall provide a tutorialistic solution to your problem.
Make a new switch and name it whatever that item name you want to use is named.
Then go database and find the tab that says comment event.
Make a new comment event, and have it be set to parallel process and set the trigger to be the trigger that you just previously made. *Name it whatever you want*
Then inside the common event make a new variable operation and make a variable named *Random Chance* and click set then look where it says random number 0 - 0 and click the marker before it to enable this action.
Then set the second 0 to 10, and press okay.
Then create a new conditional branch, and look where it says variable and then click the marker before it to enable this action ,and set the variable being determined to *Random Chance*, then set the equation to *<=*, finally to change the percentage that this will work change the constant to a number 1-10 and keep the box at the bottom checked and click okay. *The lower the number the lower the chance of it happening*
Now look for the *end battle* trigger as well as an event to turn the *item name* trigger off, and put that in the conditions meet section NOT in the else section and make sure to put the event that turns the trigger off, after the end battle trigger.
Now underneath the else trigger create a new *message* event and have it say something that tells the player that it failed or something similiar like that, and create an event to turn the *item name* trigger off.
Finally go to that item that you want to do all these actions and have it set the trigger that you made in the first step, on. Then V'alle you are finished!
*Contact me if you have any problems*