Fortitude Runeword

Fortitude (Forty) is a highly sought after runeword in Diablo 2: Resurrected. Its most notable quality is that it is possible to assemble it in a body armor as well as a weapon. While this in itself wouldn’t be particularly interesting, the fact that the stats it provides in both slots are similar still leads to some unexpected use cases. 

First of all, here are the runes and full stats it provides (for brevity’s sake, we will only look at the armor version, as that is by far the more popular pick):

        El+Sol+Dol+Lo 

 20% Chance To Cast Level 15 Chilling Armor when Struck
+25% Faster Cast Rate
+300% Enhanced Damage
+200% Enhanced Defense
+15 Defense
+X To Life (Based on Character Level)  
Replenish Life +7
+5% To Maximum Lightning Resist
All Resistances +25-30
Damage Reduced By 7
12% Damage Taken Goes To Mana
+1 To Light Radius

The most unusual stat on Forty is without doubt the +300% Enhanced Damage (or the Enhanced Defense on the weapon version). So unusual in fact, that the only other source of Enhanced Damage on a body armor is the Duress runeword. In comparison, Duress provides +10-20%, whereas Fortitude gives a massive +300%.
The +5% to Maximum Lightning Resist makes Forty incredibly useful against the most annoying enemies in the game: Willowisps (more commonly known as Souls) that one normally encounters in the Great Marsh, Act 4, or the Worldstone Keep, and Lightning Enchanted rare monsters. These monsters have no problem shredding a character even with capped lightning resist, and increasing this cap from 75% to 80% will decrease the lightning damage taken by 20%.
Bonuses to All Resistances and Enhanced Defense make Fortitude a solid choice for a body armor. Since it’s most commonly used on Act 2 Mercenaries, it can be placed in an Ethereal armor. This, along with the 200% ED can result in an armor piece with over 2000 Defense.
The +Life is also very useful, since it’s one of the few sources of +HP that Mercenaries do benefit from (in comparison, increases to Vitality do nothing for them).
Damage Reduced by a flat amount is another deceptively useful stat. While it may not seem much, even 7 can provide a noticeable boost against attacks that look like damage over time effects, but “secretly” do a massive amount of small hits instead (such as Pit Viper poison clouds).
  

Based on the stats we’ve highlighted above, we believe that the best use of Fortitude is on an Act 2 Mercenary. The defense (even without the Ethereal bug from the original D2) is solid, and it provides resistances (even +max to lightning), which is what you want from an armor. What makes it incredibly good is the Enhanced Damage, which also increases the amount of life the Mercenary leeches. 
Normally, Mercenaries die quite a lot, unless you’re very quick at killing everything on the screen. Even so, the lack of sustain makes them eventually fall to chip damage. 
Life leech is almost mandatory to run on a Mercenary to solve this; however, their damage output is usually slightly too low to counterbalance the amount they take. Since the weapon slot is usually reserved for an Infinity, Pride, or a Reaper’s Toll, it’s not really an option to swap the weapon for something more damaging to increase their sustain – the logical conclusion is that it has to come from another source: body armor.
Fortitude is very popular among characters with physical damage as well, such as Bowazons, Frenzy Barbarians, or Fury Druids. These attack-based builds benefit more from the Enhanced Damage than from the +All Skills on an Enigma or Chains of Honor. 
Since we’ve established that Forty’s other stats are also great for a chestpiece, it’s a natural pick over the other popular runewords or uniques.
  

In conclusion, Fortitude is a very good runeword for its price. While it’s not as game-changing as an Enigma or an Infinity, it could still mean the difference between spending hundreds of thousands of gold on resurrecting your Mercenary and having him happily tank hits in Hell. On the other hand, Fortitude costs about a quarter of what an Enigma would, so it’s fairly cost-efficient. In single player, a Merc Forty is usually the best place for a self-found Lo rune, given that the alternative Grief is very limited in its use, and it’s a bit too rare to Cube together 4 of them for a single Ber rune. All in all, a very solid runeword that doesn’t break the bank either.