Another one of my favorite characters in WH40k is Khârn the Betrayer. Khârn is a close combat monster when everything works, but can be blasted to pieces pretty easy.
On the charge Khârn gets 7 attacks and benefits from Furious Charge, putting his initiative and strength at 6. The special ability of his close combat weapon lets him always hit on 2+ and most everything is a 2+ to wound. All of those being power weapon means nothing much stands in Khârn's way.
Khârn uses power armor so he can use a Rhino but is probably best used in a Land Raider. Another plus of the power armor is that he can sweeping advance. With a leadership of 10 and an enemy unit going to be down a number of attacks anything that fails its morale check is going to take some hits. With the Fearless ability he isn't going to be running away from the enemy like a coward.
Another minor plus, but also a risk, is Khârn's plasma pistol. I have used him to blow speeders and other light armored items out of the sky with relative ease.
A major plus of Khârn is that he is regarded as one of the best HQs for Chaos Space Marines given his high effectiveness and relatively equal cost to a Chaos Lord.
An effect of Khârn's close combat weapon is to use 2d6 for armor penetration. With the number of attack he gets stationary vehicles are toast.
Khârn is blessed by Khorne himself and psychic attacks are useless against him. In addition to that force weapons are downgraded to power weapons against him.
By far the most glaring weakness of Khârn is his lack of Eternal Warrior and toughness 4. One unsaved swing of a powerfist in the first round of combat and it is lights out for this 10,000+ year old warrior.
Khârn is a team killer and the chances of him killing a friendly is 73% on the charge.
Khârn is primarily effective in close combat. If he doesn't get there and get the charge then the points and effort has been wasted.
The 2+ ability of Khârn's axe only works on models with a weapon skill so that Land Raider that moved more than 6" is still going to be hard to hit.
When running Khârn the biggest threat is instant death. Put him with a squad in a vehicle and make sure that vehicle gets to where it is going as best as possible. I usually use a Land Raider for the most survivability and to assault what I can when I get there.
When possible and when I know I can get an assault off I disembark the squad from one hatch of the land raider and disembark Khârn from the other. Next, the squad goes after one unit while Khârn goes after another. This ensures that the target isn't wiped out and your forces aren't left to be shot at on the enemy's turn.
Also by breaking Khârn off from the group it may be possible to target a unit that does not have a powerfist type implement. Obviously that opportunity is invaluable.
A strategy I have read about being used is to employ the use of Khârn with models that have an invulnerable save. The available units are Possessed Chaos Space Marines, Thousand Sons, and Terminators.
Terminators aren't Fearless and negate a sweeping advance and limit Khârn to the use of a Land Raider. Thousand Sons are slow and purposeful so putting an assault character with them is be foolish. Possessed CSMs are the strongest option of the 3 and can use a Rhino but suffer from their random ability tablee being mostly useless.
In my personal opinion this tactic could be viable but with proper use of tactics Khârn can be highly effective without having to buy ineffectual units.
Be smart with Khârn and he will deliver results.
Granted I haven't played this game as much as others but I have used Khârn to great effect almost every time I have used him.
Once when battling some Tau I used the tactic as described above to break Khârn apart from his squad and charge 4 or 5 crisis suits with one of them being a strong HQ choice. I chose to put Khârn's attacks into the squad instead of the commander and with a crappy roll 4 dice missed. 3 wounds later the unit failed it's leadership test and ran off the board. Khârn consolidated off the board in an attempt to attack a tank but the tank was destroyed by a supporting Defiler.
Another time in a somewhat large combat Khârn charged a squad of Space Marines. One of Khârn's to hit rolls was a 1 and the closest unit was a Defiler. Strength 6 + 2d6 later and a 6 on the damage chart meant one drastically dead Defiler.
Some say that using Khârn is too much of a risk. Killing your own models is a waste of points. Who knows, that one model could have gone on to win the game.
In an effort to see just how detrimental Khârn can be I ran a simulation using MS Excel. The simulation assumes that Khârn is on the charge and gets his 7 attacks. Using the RANDBETWEEN function to randomly pick a number between 1 and 6 and duplicating this functionality across 7 columns and 1000 rows this effectively simulates what Khârn's rolls would be on 1000 charges.
Out of 1000 charges 275 of them, or approximately 28% of the time, Khârn rolled 2+ for his to hit rolls. That also means that approximately 72% of the time a friendly model is going to take a hit.
Now the question is, "How many friendly models are going to get hit 72% of the time?" A 40% chance exists that only one model will be hit, 24% chance that two models will be hit, 7% chance that three models will be hit, and a 1% chance that four models will be hit. 5 ones being thrown in one roll is a 1 in 7,776 chance. This case happened once out of the simulated 1000 throws.
Of the 1000 charge simulations no instance exists of 6 ones rolled or 7 ones rolled so this case is infinitesimal. For 6 ones the chance is 1 in 46,656 and for 7 once the chance is 1 in 279,936. If you ever roll either one of these two go buy a lottery ticket.
In my opinion the chances of Khârn hitting friendly models is acceptable. Hard lessons learned like the one above with the Defiler go a long way to learning what tactics to use when positioning Khârn for the assault.
When using Khârn repeat the phrase, "KILL! MAIM! BURN! BLOOD FOR THE BLOOD GOD!"