So far we've looked primarily the concepts within the Grey Knights codex that fit Grey Knights. However, Grey Knights are currently unique in having a force organisation chart swap which can drastically change their outlook. This is of course with Coteaz and Henchmen. For the most part, Grey Knights are expensive Marines. They all have stormbolters. They all have Nemesis Force Weapons. Each squad can carry Psycannons, Incinerators or Psilancers (and you rarely see anything but Psycannons). Everything else is Marine based such as Rhinos and Razorbacks, Techmarines, 'fast' Marines, 'heavy' Marines, 'elite' Marines and of course a heavy emphasis is placed on Terminators. You can flip this whole concept on its head with Coteaz and henchmen as Troops.
Whilst you obviously still have Grey Knight options in the rest of the FoC, Troops becomes a completely different ballgame. Henchmen are normally Elites and can fulfil many roles. The three most common here are bringing melta to the table (Servitors, Jokerao, Acolytes), ranged firepower (Servitors, Jokerao) or counter-assault (Death Cult Assassins, Crusaders). All of these roles are excellent and generally used to plug a hole in the Grey Knight army for cheap. As Troops however, expanded roles open up.
Fire Support becomes a much better option with two or three Jokerao + ablative Acolytes holding objectives far away. Using the 1+1 concept you can still get a forward acting transport which hurts the opponent. Cheap scoring Acolyte squads can also be used. Whether it's the 12 point minimum squad size of hilarity or using six transports, six squads of six with stormbolters for cheap transport access, cheap scoring and cheap firepower, running Henchmen as Troops is a very different list for Grey Knights and many of the concepts discussed before play lesser roles. These roles are still applicable of course, especially if you are running infantry GK elsewhere (Fast, Heavy or Elite), just differently. For example, you can still have counter-assault henchmen units or units which make excellent use of stormbolters. And just because you're running henchmen doesn't mean Fortitude or other psychic powers are less effective (though less numerous perhaps).
By running lots of Henchmen as Troops, you reduce the overall effectiveness of each unit. Rather than every unit can do everything to a certain extent, you have units which need a lot more support to be able to combat effectively on the battlefield. However, these units can often be more effective at their particular role such as Death Cult Assassins, do something the normal Grey Knights cannot do (heavy weapons at 24"+ ranges) or do it for significantly cheaper. The combination of these three allows you to make a different Grey Knight list that perhaps has more toys or goodies in it than usual. Whilst it may still be midfield dominate it has potential AP1, potential ranged assault, more combat potential and more bodies overall. For an army of elite Generalists which thrive in the 24" range, all of these options are amazing to have and when they all score...well even more awesome.
There is of course the other side of the coin: using scoring Henchmen to support Grey Knights. Rather than making them the core of the list with special Grey Knights surrounding them, Henchmen as Troops can simply add more scoring potential on the cheap for a Grey Knight army. Remember, Coteaz is a great HQ by himself and well worth his 100 points without the FoC swap so it's not like you are losing anything by not maxing out Henchmen Troop squads. A couple of cheap squads can provide some AP1, ranged support and extra scoring that Grey Knights can have issue with due to the expense of their own models. 92 point scoring LasPlas Razorbacks for example or 86 point 2x Jokerao, 4x Acolyte units to sit in backfield (which can still take a transport). Having these cheap options which can help support some of the general Grey Knight weaknesses is an amazing asset.
Henchmen are not going to see use in every list but they are the black to the Grey Knight's white. They certainly can do things the Grey Knights can do (Stormbolters, counter-assault, etc.) but they do so in very different ways. They are cheap, they perform well in combat, they perform well at range, they are numerous, etc. These are all very different to Grey Knights and combining the two is going to get you a list which competes in all facets of the game and is limited by model or unit count. This doesn't have to mean Coteaz is even used or when used, that all six Troops become Henchmen. These are possible armies but again, it comes down to a balance between the two. There are pros and cons of using no to few henchmen and pros and cons to using lots of henchmen. What's important to remember is what they provide outside of Grey Knights and what weaknesses they bring to the Grey Knights army (less reliable).
