Do Diablo II mercenaries gain new skills as they level?

Question

I'm currently using a necromancer with a high distribution of skills on raise skeleton and skeleton mastery/summon resist.

To compliment this I have kept my henchman from Act II who has the Blessed aim aura, which all my skeletons obviously inherit.

Simple question: will my henchman work up to a new and more powerful aura like Fanaticism? Or will replacing my henchman in nightmare or hell maybe give him a more effective aura?

Answer 1

Mercenaries do not learn new skills as they level up, but specifically regarding the Act II mercenaries, they do have different skills in different difficulties.

See this page on diablowiki for a list of mercenary skills. An excerpt from the page regarding the Act II mercenaries:

Combat (Normal): Prayer
Combat (Nightmare): Thorns
Combat (Hell): Prayer
Defense (Normal): Defiance
Defense (Nightmare): Holy Freeze
Defense (Hell): Defiance
Offense (Normal): Blessed Aim
Offense (Nightmare): Might
Offense (Hell): Blessed Aim 

Answer 2

@Oak has this completely in the bag. One more thing to keep in mind that is that mercs are often used as an additional set of gear slots for the purpose of auras - there are some incredible higher-level runewords that grant some incredible auras. So keep in mind that you could get a fairly high-level Might or Fanaticism aura from a particular runeword when selecting which merc you want - it can be quite a pain to get a merc up to higher levels.

Answer 3

As the others have said. The mercenaries will keep the aura all game long and it only depends where you bought him. The best aura for a summoner is considered to be the might aura, because it buffs the damage of your whole squad.

Here are the formulas that calculate the aura level:

Mercenary Type                    Hlvl 9~30                     Hlvl 31~54                      Hlvl 55~98
Combat (Normal)                   3 + [(hlvl - 9) × 10 / 32]    10 + [(hlvl - 31) × 10 / 32]    18
Defense (Normal)
Offense (Normal)

Combat (Nightmare)                                               5 + [(hlvl - 31) × 10 / 32]    13
Defense (Nightmare)                                              6 + [(hlvl - 31) × 10 / 32]    14
Offense (Nightmare)*                                             7 + [(hlvl - 31) × 8 / 32]

Combat (Hell)                                                                                   16
Defense (Hell)
Offense (Hell) 

Hlvl stands for the level or your mercenary. As you can see: the aura of the Offense Nightmare (Might Aura) is not capped, but has an implicit cap of 23, if the merc reaches level 99.

High level summoners try to get the beast runeword (BerTirUmMalLum) on themselves. Eventhough they never attack with the weapon. It gives them the desired Fanatism aura.

Beast: Level 9 Fanaticism Aura When Equipped +40% Increased Attack Speed +240-270% Enhanced Damage 20% Chance of Crushing Blow 25% Chance of Open Wounds +3 To Werebear +3 To Lycanthropy Prevent Monster Heal +25-40 To Strength +10 To Energy +2 To Mana After Each Kill Level 13 Summon Grizzly (5 Charges)

In addition to the aura benefit of the a2 merc hes good because he can wear very good weapons for your squad. He can wear the pride runeword (ChamSurIoLo). It gives you a concentrate aura, which also increases the physical damage of all party members. Or Infinity (BerMalBerIst), which gives a convinction aura to increase the damage of your corpse explosion.

Pride: 25% Chance To Cast Level 17 Fire Wall When Struck Level 16-20 Concentration Aura When Equipped 260-300% Bonus To Attack Rating +1-99% Damage To Demons (Based on Character Level) Adds 50-280 Lightning Damage 20% Deadly Strike Hit Blinds Target Freezes Target +3 +10 To Vitality Replenish Life +8 1.875-185.625% Extra Gold From Monsters (Based on Character Level)

Infinity: 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)

The other great option is insight for crazy fast mana regeneration to be able to spam corpse explosion and all your curses:

Insight: Level 12-17 Meditation Aura When Equipped +35% Faster Cast Rate +200-260% Enhanced Damage (varies) +9 To Minimum Damage 180-250% Bonus to Attack Rating Adds 5-30 Fire Damage +75 Poison Damage Over 5 Seconds +1-6 To Critical Strike +5 To All Attributes +2 To Mana After Each Kill 23% Better Chance of Getting Magic Items

Keep in mind that all the above runewords can be used to cast an iron golem, that inherits the stats of the items including the auras. So in theory you could carry a Beast, your merc will carry Infinity, and your Iron Golem was made of Pride. That will give you Might, Fanatism, Concentrate and Conviction. You have to be careful not to lose the iron golem.