Thorns, Iron Maiden and Spirit of the Barbs Mechanics

Diablo II Expansion, patch 1.09d
By Tommi Gustafsson

1. Damage Return Mechanics

There are three abilities in the game that return a percentage of damage inflicted to the defender: Thorns aura, Iron Maiden curse and the aura of the Spirit of the Barbs. They work as follows:

1. An attacker must score a hit to the defender in melee combat to receive damage from these abilities. Returned damage hits the attacker automatically.
2. The damage of these abilities is calculated from the physical damage dealt to the defender, after all damage modification effects, except unit type modification and Energy Shield, have been applied. This damage amount is taken as the source damage for these abilities.
3. These abilities deal physical damage to the attacker equal to the source damage times their returned damage % / 100.
4. The returned damage is affected by all damage modification effects of the attacker, including the unit type modification and Energy Shield.
5. The damage of these abilities does not trigger any special abilities, such as Life Tap, crushing blow, or sorceress's cold armors, except the blood golem's life leech ability. This is the only ability which is triggered due to Thorns or Iron Maiden damage.

Note that damage from these abilities is reduced by the physical resistance of both the defender and the attacker.

2. Related Questions

2.1 Do Thorns, Iron Maiden and Spirit of the Barbs Stack?

Yes.

2.2 Who gets the kill if a creature is slain due to Thorns, Iron Maiden or Spirit of the Barbs?

The creature that was attacked is counted as the killer. Thus, if a creature cursed by Iron Maiden kills itself by attacking a neutral unit, such as Bone Wall or Bone Prison, no one gains experience.

2.3 Which one is more effective: Thorns + Amplify Damage or Thorns + Iron Maiden?

Necromancers may use both Thorns mercenary (act 2 combat mercenary in nightmare difficulty) and Iron Maiden curse. However, Thorns damage can also be enhanced via Amplify Damage curse, which is mutually exclusive to Iron Maiden. Thus, the question is which combination is more effective: Thorns + Iron Maiden or Thorns + Amplify Damage?

Let's assume the following skill levels:

• Thorns aura at level 17, which returns 890% of damage. This is the maximum level of Thorns for a mercenary. His aura reaches this level when the mercenary gets to level 55.
• Iron Maiden curse at level 30, which returns 925% of damage. This is reached by maxing Iron Maiden and wearing +10 skills items.

Note that monsters have typically 0% physical resistance in nightmare difficulty and 50% in hell difficulty. Also, Amplify Damage curse lowers physical resistance by 100%. Thus, it typically doubles damage from Thorns in nightmare difficulty and triples it in hell difficulty.

Now, we can calculate the combined effects of Thorns and the two curses. It depends on the physical resistance of the attacker and the defender.

Physical Resistance1)		Total Returned Damage %
Attacker2)	Defender	Thorns + Amplify Damage	Thorns + Iron Maiden
0% Nightmare monster	0% Blood golem	1780 % (2*1*890 %)	1815% (1*1*(890%+925%))
0% Nightmare monster	50% Resistant revive	890 % (2*0.5*890%)	907.5% (1*0.5*(890%+925%))
50% Hell monster	0% Blood golem	1335 % (1.5*1*890 %)	907.5% (1*0.5*(890%+925%))
50% Hell monster	50% Hell revive	667.5 % (1.5*0.5*890 %)	453.75 % (0.5*0.5*(890%+925%))
100% PI monster	Any	0 %	0 %
Any	100% PI revive	0 %	0 %

1) Before the effect of Amplify Damage.
2) The attacker is the target of Amplify Damage. The curse lowers its physical resistance by 100% (except if the attacker is  immune to physical damage).

In nightmare difficulty, both options are about equally effective. However, in hell difficulty, Thorns + Amplify Damage is a lot stronger. Thus, if one uses Thorns mercenary, Amplify Damage is usually a better choice than Iron Maiden. Iron Maiden comes into question when the mercenary is dead or his aura has not been activated. Amplify Damage has also an advantage of increasing any physical damage done to the target, plus it has a larger area of effect.

3. Acknowledgements

Bradosol for pointing out that Life Tap does not work with Thorns and Iron Maiden.
Jarulf for his article on damage order calculations and information about how Thorns interacts with damage modification effects.
Kalil for reporting that Thorns and Iron Maiden stack.
Nicodemus Phaulkon for explaining that no ability, other than the blood golem's life leech, is probably not triggered due to Thorns or Iron Maiden damage.