Since it grants Thorns, enemies will take damage every time they land a hit on your character, whether it deals damage or not. This makes this relic especially effective against enemies that relies on multiple-hits attack such as Book of Stabbing, Byrds, Snake Plant and Corrupt Heart.
Also, since enemy's Block fades at the start of their turn, it means that they have no way to prevent Thorns from directly damaging their HP.
The flavor text says that these scales belong to The Guardian. However, The Guardian's Sharp Hide buff works differently than Thorns, dealing damage to the player per attack card played instead of per hit landed.