Infinity Runeword
    
Diablo 2: Resurrected’s Infinity runeword is one of the most expensive and highly sought after piece of equipment. Normally placed on Act 2 Mercenaries, it provides a Conviction Aura that can make Elemental Immune monsters susceptible to damage, thereby allowing several builds to properly function in places where they normally couldn’t.

Firstly, let’s look at the properties of the runeword before diving into the details:
        
        Ber+Mal+Ber+Ist
        
50% Chance To Cast Level 20 Chain Lightning When You Kill An Enemy
Level 12 Conviction Aura When Equipped
+35% Faster Run/Walk
+255-325% Enhanced Damage
-(45-55)% To Enemy Lightning Resist
+40% Chance of Crushing Blow
Prevent Monster Heal
0.5-49.5 To Vitality (Based on Character Level)
30% Better Chance of Getting Magic Items
Level 21 Cyclone Armor (30 Charges)

Going through the stats, the most notable one is the Level 12 Conviction Aura When Equipped. This lowers the elemental resistances of the affected monsters by 85. “Elemental” is important to note, as Conviction does not lower Magic or Poison resistances. (Sorry, Hammerdins & Necromancers!)
While 85% sounds massive, due to the somewhat complicated way enemy resistances are calculated, it’s not always quite that much. Nevertheless, Convi might be even more useful when it doesn’t lower resistances by the full amount.
Without going into massive detail on the various resistances of every monster type in the game, we’ll provide some “ground rules” that will give you an idea of what a level 12 Conviction can do. 

Firstly, it is important to know that Hell difficulty is full of monsters that are immune to a certain damage type. In practice, it looks like this: the monster gets an “Immune to XY” tag if its resistance to that damage type goes above 100%. This could be because they naturally have that much, or because they are Champions or Rares that receive additional resistances from an affix such as Fire Enchanted or Stone Skin. 
To remove the “Immune” tag and make them susceptible to damage, one has to lower the monster’s resistance to below 100%. Conviction’s one caveat is that if it has to break an immunity this way, it will only function at 1/5 of its original efficiency – so in our case, it will lower immune monsters’ resistances by 17.
Secondly, the resistances of Immune monsters are varied. As a rule of thumb, the Lightning Resistance of Lightning Immune monsters is not much higher than 100%, while Fire and Cold Immunes have higher Fire/Cold Resistances. 
With this in mind, we can say that Conviction is very good for breaking the immunities of Lightning Immune monsters, but leaves a fair amount of Fire and Cold Immunes untouched.

After this slight tangent on Conviction and immunities, let’s look at the remaining stats that Infinity offers. 
The amount of Enhanced Damage we get is relatively high, even among the top tier weapons. This, paired with the 40% chance for Crushing Blow (CB) provided by the 2 Ber runes make Infy a solid weapon that makes the Mercenary equipping it capable of killing at a decent speed on his own. CB in particular makes it a great help against bosses with a high health pool. 
The -45-55% to Enemy Lightning Resistance is definitely worth mentioning as well, but unfortunately this stat is a bit misleading. While some people place value on it, unfortunately this is not an overall reduction of the monsters’ resistance like Conviction. -ELR is only taken into account when the character holding the weapon is doing Lightning damage to the monster. Therefore if the Mercenary of a Lightning Sorceress is using an Infinity, this stat won’t reduce the monsters’ Lightning Resistance against the Sorceress’ damage. 
The rest of the stats aren’t particularly noteworthy as they either flat out don’t work on a Mercenary (+Vitality, Cyclone Armor charges, Faster Run/Walk), or aren’t really beneficial or interesting.

Since Infinity’s biggest draw and unique selling point is the Level 12 Conviction Aura, it is safe to assume that whatever builds benefit from Convi the most are the ones that will want to run Infy the most as well. From our look into immunities, we can say that these are characters that do Lightning damage primarily: Lightning Sorceresses, Javazons and Trapsins. These characters will most likely see Lightning Immunities broken across the board, with a few Champion/Rare exceptions that the Mercenary can usually deal with.
Even though Infinity doesn’t necessarily help with immunes against their damage type, other builds will still most likely run it as well. For instance, the resistance reduction is very handy against bosses for everyone, and as we’ve mentioned, Crushing Blow is great as well. 
The only real alternative is Reaper’s Toll, which is similarly useful for Physical Damage builds since the Decrepify it procs can break Physical Immunity.

Being a runeword, players have a choice in what item base they put their runes in to make an Infinity. We’d like to avoid whipping out the Frames per Attack tables and weapon damage calculators, and will instead just offer a few pointers about the various polearm bases. 
Thresher and Giant Thresher (GT) are widely considered to be the best bases, because they are much faster than the alternatives while offering similar (slightly lower) raw damage output. While having no noticeable downsides, the Thresher’s rapid attacks offer 2 benefits: they proc Crushing Blows faster, and they make Life Leech more consistent, which makes Mercenaries much more durable. GTs, unfortunately, have a massive Dexterity requirement, which makes them impractical to use on a Mercenary.
Cryptic Axe (CA) and Great Poleaxe (GPA) are the next best options. At certain IAS breakpoints, it will either be CA or GPA that deals the most raw damage per second, but the slower attack speed (relative to the Thresher and GT) makes them a bit less desirable.
Colossus Voulge (CV) is another very popular option. Unfortunately it’s not particularly great at anything. It has the highest Strength requirement of all polearms, and it is also the slowest tied with CA. It has a massive upside, however: CVs are the only elite polearms that have a maximum socket number of 4. In practice, this means that you can simply take a CV to Larzuk and he will put the guaranteed 4 sockets into it. They also have a 50% chance to get 4 sockets from the Horadric Cube socketing recipe. On the other hand, all other elite polearms have to either drop with 4 sockets, or have to be Cube socketed – but they only have a ~17% chance to come out with 4.

All in all, Infinity is a runeword that’s definitely worth its cost of 2.5 High Runes. Immunities are incredibly oppressive in Hell difficulty, and just the ability to circumvent them on certain builds is priceless. So much so, that Infinity (along with Insight) is mostly the reason why Act 2 Mercenaries are so heavily picked over others.