Let's start with comparing their statline with that of the standard Marine. Firstly of course they have +1 Toughness. This makes a huge difference to survivability against small arms fire, halving the wounds taken from S3 weapons. Even Fire Warriors only have a 50/50 chance of wounding when they hit. Note that this bonus is ignored for the purpose of determining instant death - which matters because of the Feel No Pain rule (see below).
To compensate, they take a penalty of -1 Initiative. This is significant, as it means that other Marines hit you first and that lesser soldiers usually get a chance to hit you simultaneously. They will take more hits in close combat than is usual for a marine - but this is mitigated by their amazing survivability (and blight grenades - see below).
Finally, they have the Leadership 9 and Fearless that is shared by all the cult Chaos Space Marine troops. With the fearless rule you will almost never need to take a Leadership test anyway. The great thing about Fearless is that it means you can rely on Plague Marines to always hold an objective - they will never run off it from shooting in the final turn of the game. Your opponent has to kill them all - and with Plague Marines that is a lot of shooting. In close combat they take extra wounds if they lose combat - but like Terminators they only have a 1/6 chance of not saving against it.
As if Toughness 5 wasn't good enough, they come with the Feel No Pain rule, giving them an additional 4+ save against most wounds. This combination makes Plague Marines three times more difficult to kill with bolter fire than a standard marine. Make sure that you understand the details of this rule: you can't take the save against any attack that would cause instant death (such as S8 krak missiles), or any attack that no model is allowed a save against (AP1 or 2, rending attacks, power weapons etc.). Plague Marines are the toughest thing to kill in your Chaos Space Marine army. The combination of Toughness 5, 3+ save and Feel No Pain is awesome.
Now is an appropriate time to talk about Plague Marines and cover. Cover is as important if not more important to these guys than other units in the Chaso Space Marine army. Why? Because their specialisation is in not dying, but AP1 and 2 weapons, and krak missiles, ignore all the extra abilities you have paid so much for. You must keep them in cover so that they can do their job (frustrating the enemy while you thank the blessings of Nurgle) throughout the entire game. Unless you want your opponent to be shooting his anti-tank at them instead of your Rhinos of course!
Like the standard Chaos Space Marine, Plague Marines come equipt with power armour, a bolter, a bolt pistol and a close combat weapon, and frag and krak grenades. In addition they have blight grenades, which count as defensive grenades. Defensive grenades remove the bonus attacks that an enemy gets from assaulting you. There are three rules complications with defensive grenades that should be addressed at this point:
- Certain units get multiple bonus attacks when assaulting (eg Blood Claws). They do not get any of these bonus attacks when assaulting Plague Marines.
- Defensive grenades have no effect on other bonuses that a unit gets for assaulting, such as the +1S +1I from the Furious Charge rules.
- If a Plague Marine assaults a unit with the Counter Attack rule, then the assaulted unit does not get the bonus attack from the Counter Attack rule. This is controversial, but the Counter Attack rule says the model gets +1 A "exactly as if they too had assaulted that turn". My point being that if they had assaulted that turn, they wouldn't have got +1A. When you're playing a Space Wolf or Straken Guard opponent, make sure that you clear this up at the beginning of the game.
Plague Marines are the only power armoured troop squad in the game that can take two special weapons in a five man squad. This is especially powerful when you consider the 2 fire points of a Rhino - extra weapons and extra bodies are a waste when you are snug inside that protective box. So what can they take? The standard imperial fare: melta, plasma or flamer. This option is so good that I have never taken a Plague Marine squad without two special weapons.
Firstly, always choose the same special weapons for both marines. There is really no exception to this, and its a rule that applies to most units in 40k. Each turn you want to maximise the impact your squad has on the game. That second weapon makes a big difference in reliability. Trust me! I often see people take 1 melta and 1 flamer. 1 flamer can be effective against basic troops in cover. However 1 melta is not enough to be considered serious anti-tank - I'd rather take the second flamer so that the squad can potentially put a serious dent in a cheap and nasty horde unit. Don't water down the unit's effectiveness in its primary role.
Plasma and Feel No Pain are a marriage made in the warp. Plasma is generally considered overpriced in 5th edition, but the extra chance to save against the Gets Hot rule makes the plasma gun worth its points in the hands of the Plague Marine. Plasma suffers from the number and strength of cover saves available, but is still a very effective weapon, able to threaten marines on foot and light vehicles like the Rhino. It is an especially strong choice in low point games, where there are less vehicles and those that exist will usually have lower armour. As it has the longest range of the weapons available to you, it is the obvious choice for a unit on foot.
Melta is the of course the special weapon of choice in 5th edition, and Plague Marines are a great way to deliver it. Vehicles are hard to kill and a couple of melta guns have the potential to cut through the toughest armour like butter. Unlike other units that deliver melta guns into enemy lines, Plague Marines have a good chance of standing up to the contents of whatever transport you just blew up. Two meltas have approximately a 50% chance of destroying a Rhino when 6" or less away.
Flamers are the final special weapon available. It's a more specialised weapon, great for digging troops out of cover and for thinning out larger units of horde troops before they overwhelm you. They can be a useful tool to have in your toolbox, but you won't need more than one flamer equipped unit in your army. Use them if you are concerned about your ability to handle horde armies with lots of foot troops (eg Tyranids).
Oh yeah I nearly forgot - you could take 2 plasma pistols instead of special weapons. Never do this. Plasma pistols suck.
The Rhino is ideally suited to the nature of the Plague Marine unit. As I have mentioned above, the 2 fire points matches the two special weapons they can carry (Note: the codex says that the Rhino has only 1 firepoint, but this is a misprint and corrected in the FAQ). Secondly, the Rhino provides a great deal of protection to the unit inside it. When that unit is Plague Marines it becomes incredibly difficult to kill. Thirdly, all the weapon options for the unit work best up close within 12" of the enemy - and the Rhino can get you there. Finally, the Rhino can take a combi-weapon that can complement the weapons chosen for the unit - such as a third melta shot when you move up into the killing zone.
More than anything else, the purchase of a Rhino will define the role of your Plague Marine unit. Without it, the unit is really only able to sit in your deployment zone and hold an objective. Without long ranged weaponry, this might result in the unit taking little part in the battle only to have the objective contested in the last turn by a fast moving unit it couldn't reach. I won't go into detail about the Rhino's options here - it deserves an article of its own and this is getting long enough already!
A difficult decisions to make when building a Plague Marine unit is whether or not to take a Plague Champion. Purchasing a Champion gives you an additional attack in close combat and an additional (basically useless) point of Leadership. The only time a plain Champion makes any sense is if you have also purchased a Greater Daemon. It is also not worth it to buy the Champion just for the chance to get a combi-weapon. That is what the Rhino is for. For the points I would much rather take an additional body.
What the Plague Champion does provide is the chance to add a power fist to your squad. The power fist gives the chance a much better chance against walkers in close combat, otherwise any squad that gets in combat with a walker will be very lucky indeed to survive (krak grenades mean you are hitting on 6s, glancing on 6s, with one attack per model). They also given a good chance to lay some wounds down on a monstrous creature. This is what you buy them for. Power fists do also give a good chance of busting up other vehicles, but krak grenades allow your normal marines to do so anyway. So - only take a power fist on a squad or two, and only if you don't have enough other units that can handle walkers for you. He makes the unit a lot more expensive.
Note that if you shell out the points for a Plague Champion, he can also take a combi-weapon (either matching the squads load out, or perhaps a combi-flamer). A combi-melta pushes your chance of killing a Rhino up to around 65% - but you need to get out of the tank to use it. My motto in these situations is "never get out of the tank", so I'd usually suggest that there is somewhere else in the list your points could be better spent - or maybe take a combi-flamer for late game objective mop up duty.
There are other weapons that a Plague Champion can take (power weapon, plasma pistol, twin-linked bolter, melta-bombs) but none of these are very effective (especially the I3 power sword) and should be left at home.
Finally the personal icon is used to allow deep strikers and daemons to enter near your unit without scatter. You can use this to drop a screen of daemons down to protect your plague marines from assault. Whether or not you take it depends on the other selections in your list. Note that it doesn't need to be given to the Champion - any marine can be equipped with it.
There is a variety of ways to build an effective Plague Marine unit - you could spend anywhere between 115 and 400 points on the squad! Here are some of my preferred combinations.
The Melta Squad (180)
5 Plague Marines with 2 meltas, Rhino with combi-melta
3 melta shots without getting out of the Rhino. Once the transport gets busted they can move towards objectives or control the movement of enemy vehicles in the vicinity. You should take at least 2 of these.
The Assault Squad (322)
9 Plague Marines with 2 meltas, Plague Champion with power fist, combi-melta, Rhino with combi-melta
Just enough room left in the Rhino so that they can be joined by a Chaos Lord or Sorcerer. This squad moves forward and can successfully assault enemy close combat specialists. Even on foot it can survive several rounds of intense shooting whilst delivering its payload if you use cover correctly. You could switch the Champions combi-melta for a combi-flamer if you are better than me at hitting with meltaguns!
The Horde Thinning Squad (216)
7 Plague Marines with 2 flamers, Rhino with combi-flamer
Ideally they tank shock one side of a horde unit to bunch them up, then jump out of the Rhino and toast them with the three flamers.
The Cheap Objective Holder (145)
5 Plague Marines with 2 plasma guns.
If you can't find the room in your list for that final Rhino, then this squad can do a good job of holding an objective in cover in your deployment zone. They can even provide fire support once the enemy gets into your lines.
Thanks for reading this far. There is plenty of army level strategy I have intentionally left out. I may publish an article on pure Nurgle Chaos Space Marine tactics in the future. I hope that my thoughts have perhaps led to you rethinking some of the strategies and load outs you have been using with your Chosen of Nurgle!