So what the heck happened to cause this change of heart ? That is what I really want to know. Maybe Kharn simply decided to fully embrace his dark side. What's interesting to me is that there were other World Eaters such as Captain Argus Brond that remained loyal to the Emperor. I don't ever remembering reading anything from GW or the Black Library that explained his decision or delved into his specific situation. Certainly Angron must have played a big part in it... That much we know for sure.
So on the tactical analysis now...
I'd like to discuss how I use Kharn in my most current Chaos Space Marine (CSM) army. Kharn has received quite a few big buffs since the last codex and received a reduction in points (160 now versus 165 points now).
2+, Better than a Wolf Tooth Necklace
As before he always hits on a 2+ in melee plus he has the Warlord Trait Hatred Incarnate which is Hatred versus everything! The newly revised CSM FAQ from GW tells us that Kharn rerolls to hit on any 1s during initial assaults due to his Hatred. This is really good as he won't be chopping up his compatriots and can inflict even more damage... Kharn attached to a dedicated melee unit (e.g., Berserkers, Chaos terminators or Chaos Space Marines) can destroy many enemy units during the initial charge so the chances of him slaying fellow World Eaters beyond the first round of close combat are slimmer now and obviously that's a good thing.
Kharn is base S5 and his favored melee weapon Gorechild, which is a Chaos Artefact, grants him +1S and is AP2 plus it has the special rule Armor Bane (add 2d6 for armor penetration). Kharn also gets to swing his axe at his normal initiative (I5) unlike other power axes... This is very good as well.
Like I said above you should attach Kharn to a dedicated melee unit. This makes him more survivable over the course of a game and he enhances friendly units such as Chaos terminators and Chaos Space Marines since they benefit from his Fearless and Hatred Incarnate special rules. Also it is worth noting if Kharn is chosen for your primary detachment he then makes Berserkers count as a troop choice which is also quite useful.
I currently prefer to use Chaos Space Marines with the Mark of Khorne (MoK) and Icon of Wrath (IoW). MoK grants the special rules Rage and Counter Attack so equipped as such the squad is actually better than Grey Hunters... Of course you're paying the points for it. IoW grants them the special rule Furious Charge and allows the squad to reroll their charge distance - that ability alone is well worth the price for this option.
This unit can be configured just like a squad of Grey Hunters - bolter, bolt pistol and a close combat weapon (commonly referred to as the uber grit) plus they have access to special weapons unlike Berserkers. I prefer to take a flamer since mass infantry is very popular now. Plasmaguns are a bad choice since they can't charge if they fire it. The meltagun is one shot and while it packs quite a wallop your goal should be to engage them in melee... The flamer helps to soften enemy units prior to the charge.
I'd like to experiment using a squad of Chaos terminators in the future... They are pricey though compared to Khorne Marines and don't score either. Still I think Chaos terminators could work well in the right list, such as if you're using cultists for your troops.
Chaos Land Raider
I prefer to run my squad with Kharn in a land raider equipped with a Dirge Caster (prevents enemy units from using Overwatch), dozer blades and extra armor. Both the Dirge Caster and dozer blades are made of pure win... I will even go so far as to say they are both mandatory. If you're feeling a bit daring you might want to use Daemonic Possession instead of extra armor... I would much rather have BS4 for the land raider two twin linked lascannons. It doesn't matter much as the game progresses though since you'll be moving the transport a lot, which will force you to snap shoot those lascannons.
Chaos Storm Eagle
If you can use Forge World then I think a Storm Eagle is the best way to go and it is superior to the Chaos land raider since it is a flyer with an assault ramp and has the capacity for up to 20 models! This is where taking a large squad of Khorne Marines or Berserkers can really pay off (i.e., 15 or more total models embarked inside the Storm Eagle... Take another flamer if you're running Khorne Marines). The Storm Eagle has the Vengeance Launcher which is excellent for trashing enemy infantry such as big blobs hiding behind an Aegis Defense Line plus you can take an option for two twin-linked lascannons or four Hell Strike missiles. You can also replace the nose mounted heavy bolter with a Reaper autocannon. I prefer the lascannons and Reaper autocannon configuration since these ranged weapons are excellent versus enemy flyers.
Dreadclaw Drop Pod
You also have access to the Dreadclaw drop pod if you can use Forge World. This transport is a cheap flyer with AV12 all around - it also has an assault ramp and frag launchers as well. The Dreadclaw can only carry 10 models and has no ranged shooting but you're saving lots of points as compared to the Storm Eagle... You can actually take three Dreadclaws for the price of one Storm Eagle - definitely something to consider as this easily lends itself to a very mobile assault army.
Note on the Rhino
I don't see the rhino as being a good choice. You can't assault the same turn you disembark and the rhino just seems so flimsy now. The last you want to happen is Kharn's
transport shot out from under him before he is in range to launch an assault. If you don't want to pay the price for a land raider or Storm Eagle and can use Forge World then the Dreadclaw is your best option.
This is another option but I never use it. It can work though and the points you save eschewing the use of transports can go towards fielding more assault units or simply running them in bigger units. Your army is slower though and will be exposed to a lot more enemy shooting as it moves forward across the battle field to engage the enemy. You can use an Aegis Defense Line deployed at the boundary of your table half to provide the 4++ cover save as your army advances.
Kharn is truly a melee beast but we need to consider his weaknesses as well so we can better protect him. Kharn's major shortcoming is his resiliency - he is base T4 and has only a 5++ invulnerable save. This is another good reason to attach him to a bodyguard. Use the squad champion to issue or accept challenges so Kharn can inflict damage into the enemy unit rather than wasting his uber attacks versus say an enemy character armed with a storm shield. This will invariably produce better results overall in melee and you'll sweep enemy units more often.
Blessing of the Blood God
It would have been so nice if Kharn had a 2+ armor save and was an Eternal Warrior but at least he does Deny the Witch on a 2+ which is very helpful seeing how popular psykers are now with their many deadly maledictions. Kharn is also immune to instant death from force weapons and a lot of people believe this includes Nemesis daemon hammers as well.
The Big Picture
I've posted some of my Khorne army lists here before and you can also find them over on my blog (Terminus Est - greenblowfly.blogspot.com). Kharn and his retinue are one of the corner stones of my army. I use daemonic allies to bolster my numbers and I'm surprised not to see more players doing so. Daemons are an excellent ally for Khorne and there's lots of solid Khornate daemonic units to choose, such as a Bloodthirster, Saemon Prince, Heralds and Bloodletters. I prefer to run my Bloodletters in packs 16 strong so they can soak up the damage on the way in. I have also used Khornate daemons as my primary detachment with CSM as their ally - it's brutal and often I find the daemons to be more resilient.
It can be quite the challenge to play an assault army in sixth edition but once you're stuck in the enemy can quickly wither... You don't see lots of assault armies now, as bad as assault is now with all the many nerfs they are still a solid contender in my opinion and I do love to exploit the meta.
You're not going to have a lot of shooting so you'll need to make the most out of what you do have. This is another reason why I prefer transports such as a Chaos land raider or Storm Eagle since they both have some serious long ranged shooting.
Kharn is pretty much a point and click über melee character... He is brutal in assault (obviously) so get him into close combat - he will take care of the business. The land raider is your best option as a transport if you have to keep him on the ground... Consider taking a Chaos Storm Eagle or Dreadclaw if you can use Forge World. Kharn needs a retinue to help him survive the battle longer. Finally daemons are an excellent ally so use them.