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.
Block: Protects yourself from damage while dealing damage against enemy hits.
Stacks fully with Caltrops, increases the potency of Thorns even further.
Complements nicely with Flame Barrier, which itself grants a hefty amount of Block to prevent damage and has an effect similar to Thorns tied to it, increases the damage against enemy hits even further.
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.