How does Thorns work?

Solution 1:

After doing some math, I arrived at the following formulas for thorns damage:

Regular Attack

• ((S / 100 / 4) + 1) * T

Reflective Skin Attack

• ((S / 100) + 1) * 2T * (P / 100 + 1)

where

• S is Strength (or mainstat)
• T is your thorns damage
• P is your Physical Damage bonus (as a percent)

It does not appear that regular thorns damage can crit, nor is it based off your weapon damage or the damage done to you. It did not to appear to be affected by any damage reduction modifiers nor is it improved by bonuses to Physical Damage Skills. It is scaled according to your mainstat by 1/4

Reflective Skin does indeed double your base thorns damage. However, because it is a skill doing the damage and not the thorns mechanic, it scales off your mainstat like all other skills (not at a 1/4 ratio). It is also then eligible to crit and receive your "+X% Damage to Physical Skills" bonuses. This is the reason for the (more than 2x) large spike in damage when using Reflective Skin.

An example:

For a crusader with 5000 Strength and 10000 Thorns and 20% Bonus to Physical Skills:

• A regular attack would deal 135,000 damage.
• An attack while Reflective Skin is active would deal 1,224,000 damage (with the option to crit).

Solution 2:

After doing some research, I have reached a few conclusions, plus found how the approximate damage works.

Thorns return damage does not depend on how much damage the attack against you does. It's calculated by your base thorns times a multiplier based on your primary stat. The multiplier is one quarter of the bonus provided to your attacking damage. For example, if you have 20,000 thorns, and 6,000 strength, then you'll get 1500% bonus thorns damage (6000 / 4 as a percentage), which amounts to 320,000 thorns damage to enemies when they attack you (20,000 base plus 300,000 bonus from primary stat). This can then crit based on your normal crit chance and crit damage.

All of the insanely high damage numbers occur using the Iron Skin skill with the Reflective Skin rune, which appears to be bugged currently and do lots of wonky stuff (including killing allied Crusaders).

Solution 3:

I did some testing because I was wondering the same question and came to the conclusion that only your main stat and dmg taken debuffs on the target affect thorns damage (dmg buffs on yourself do not increase thorns dmg, atleast not the ones I tested with), dmg taken debuff affects thorns dmg by 75% the normal amount and main stat increases thorns dmg about 25% of the normal dmg increase.