How about making that Item as an Upgrade/Research for it to become available ?
This way, you can even name your Research something like "You have to defeat Baltrossa first to unlock this item" or something like that to acknowledge the players on what they should do to get the item.
And of course, after you do certain Events (based on the item's requirement), set the Upgrade tech level to 1 (or higher than that - to enable the item).