This can be used on a weapon or armor to mimic the upgrade rating, gems and enchantment and transfer it to another weapon or armor.
You can buy a mimic scroll from the Token Exchanger in the Depot. The cost is 50 Archlight Tokens. Mimic Scroll once used on a piece will change the way it looks.
Gif explaining how to mimic stuff from one item on to another.
Mimic scrolls only transfer whatever is on the SCROLL. What this means is that if you have a weapon with upgrade +7 and a mimic scroll with regrade level 4, using the scroll on the weapon will only open the 4 regrade slots.
HOWEVER If the mimic scroll has a different upgrade (lets use +3 as an example) + regrade, then the upgrade that is already on the weapon will be lost.
Mimic Scrolls can be used on your gear, soul runes and healing emblems.