For some reason it appears that Imperial Guard are the allies of choice for just about anyone who can take them. While I like the idea of traitor Guardsmen, there is something indescribably compelling about bringing along a detachment of Chaos Space Marines to the battle to guide the Daemonic infestation.
I am going to lay the groundwork to work through the entire CSM codex in this post and I will start with the base requirements for an ally - HQ and Troop selections (I will only discuss troops today and move to HQ next). In selecting these units there are two criteria for allies I am going to discuss. First, allies need to bring something to an army that it cannot do well on its own. Second, allies need to complement the playstyle of the base army.
In my experience the major weaknesses in the Daemons codex includes:
1 - Expensive troops that must be left in reserve,
2 - Poor ability to destroy vehicles in the shooting phase,
3 - No psykers to act as force multipliers,
4 - Force fragmentation due to required reserves
5 - Difficult to take advantage of fortifications,
6 - Difficulty dealing with flyers.
Although Daemons have benefited significantly from the rule changes between 5th and 6th edition in many ways, three of the six weaknesses enumerated directly relate to their difficulty interacting with the new 6th edition rules. Luckily, as the first codex of 6th edition Chaos Space Marines (CSM), they were designed to some extent with these rules in mind. They are also the only Battle Brothers for Daemons - which is a bit of a moot point compared to the other available allies because they cannot join/be joined by Daemon units. Keeping these weaknesses in mind, and dealing with the most significant first, the CSM troops section offers some great opportunities to resolve a few of these shortcomings.
There are really two options for an Allied CSM contingent with troops - remember the cult marines can only be unlocked with a primary detachment including the appropriately marked Lord/Sorcerer - Chaos Space Marines and Cultists.
Chaos Space Marines are very reasonably priced - 2 points less than a plaguebearer stock - and provide a MEQ statline. They can take the normal complement of ranged weapons (melta, flamer, plasma) and have a heavy weapon option too. There is a very specific reason, however, that I do not like them as an option for a Daemon Ally - they bring a MEQ stat line to the list. Do not get me wrong, marines are good and these marines are cheap, but Daemons dodge a lot of firepower by presenting a hugely different target profile from most other units. Vindicators, Basilisks, Plasmaguns and a variety of other weapons lose a significant amount of their luster when there is not a 3+ to bypass.
The addition of Cultists to the CSM codex is one of the most underrated additions to the new book. Cultists provide scoring bodies for 9 points less than a CSM with a typical guard statline. The come default armed with a pistol/ccw and can be taken in squads of up to 30. This makes them the perfect backfield objective camping unit - 20 bodies for under 100 points is not something Daemons are used to having. They are also perfect for manning an Aegis Defense Line with a Comms relay to guarantee most of the Daemon primary detachment will arrive by Turn 2. Their weakness in Close Combat (or shooting for that matter) is less relevant for Daemons who can put a large number of deadly bodies between the cultists and enemy troops.
When taken with an Aegis, cultists also avoid the inherent vulnerability of any Daemon ally - being left alone to face enemy shooting on Turn 1. In most cases their 2+ from going to ground will be enough to keep them healthy - and with Night Fight in half of games and going first in half of games, they will be strongly protected in 75% of games. That said, the major weakness for Cultists is not being shot off the board - it is failing a leadership test and running off of it. Addressing this problem requires appropriately sizing the squad and attention to the HQ section to select a choice that preserves their ability to Go to Ground behind an Aegis while bolstering their leadership to prevent them from fleeing the moment bullets start to fly.
While fearless offers cultists the opportunity to stay put and get slaughtered to a man, it also greatly reduces their survivability behind an Aegis or in Area Terrain. Generally speaking, Daemons have the luxury of ignoring leadership completely with universal Fearlessness and taking a non-fearless unit goes against the conceptual unity of the army - but I think it is important to utilize an Aegis to its maximum potential.
Before moving to HQ units, I am interested in some feedback on my argument against CSM as a troop selection and against taking a fearless HQ!