Well, this article probably could just write itself, if it even needs to be written at all. But for the sake of completeness, I'll have it up here anyway. Molik Karn is considered our best beast, and some people consider it one of the best beasts in the game. It's all about offense, and it does its just pretty dang well.
To start, Karn has the best SPD of all Skorne heavies- he's as fast as the Skorne lights. His STR is the same as other Cyclops, but he has a great MAT of 7. His DEF/ARM is 13/18, which isn't terrible, but he's pretty frail given how much attention will be paid to him. He has 28 health boxes, which isn't great, 8 CMD which is handy given one of his special rules, and has 4 FURY/10 THR which means you're going to be able to do a decent amount with him in a single activation. He's the most expensive of the Skorne beasts (actually- of any Skorne model at this point), so given how fragile he is, he's not going to be showing up in every list. It's dependent on whether you'll actually have a good use for him.
Karn's attacks are two P+S 13 Falchions. They have Reach, which is extra handy given Karn's mobility, and Weapon Master. Yes, a beast with Weapon Master. Combined with the various ways Skorne can buff their beasts, this has some pretty scary potential.
To top it all off, the Falchions can be used to Combo Strike as a special attack, giving you an option for a single P+S 18 attack with weapon master. Sometimes it might be worth doing this, especially if you're doing some of his various tricks. We'll look at that in a bit.
His animus is Fate Walker. After all friendly models have finished their activations, models who have had Fate Walker used on them (the animus has a range of Self) can make a full advance. There are two uses for this- first, you can use it on your warlock to get him into either a better position, or a safer position. Having the option to move after seeing how your turn has unfolded is a fantastic option. Second would be to move Molik Karn. Since this doesn't negate free strikes, you'll most likely use this after a Side Step, or when Karn is out by himself. Getting him into position, or locking up the enemy warcaster/warlock, or getting Karn around incredibly fast are all options here- it's an animus that will see a good amount of use.
Affinity [Makeda]- +1 FURY while in either Makeda's control area. That's an AWESOME affinity. 5 FURY can come in handy so often.
Chieftain [Cyclops]- Friendly Cyclops within 8" of him become THR 10 instead of 8/9. If you're running a few Cyclops, that's a HUGE help for leaving a fury or two on a beast.
Future Sight- Just like the Cyclops Savage, he can boost after rolling. Great for making the most of his fury, and with MAT 7, you'll hit a lot of things anyway, so you can get a lot of mileage out of him. Extra handy for his 'bullet' stuff.
Intuition- Stolen from the Cyclops Brute. On a DEF 13 model, I just don't see this being used all that much. Maybe if your opponent happens to hit with one of those inaccurate AOE attacks with a lucky roll. Otherwise don't bother.
Side Step- If Karn hits with either his initial attacks or a Combo Strike, he can move 2" without fear of free strikes. This is a big part of Karn's mobility. He has some versatility here, since he can either use it once or twice (based off if he Combo Strikes or not), and there are a few uses. It can be used to get him out of combat so that he can Fate Walker away. It could also be used to reposition himself deeper into the enemy lines if he was going for the caster-kill, or some other really important target.
Archdomina Makeda: Where to start? The Affinity is the first thing to notice here. Having five available fury can come in handy several times. That's a great start already, but her spells make Karn even better. Carnage is nice, but Karn doesn't really need it, but obviously won't complain if he benefits from it. Defender's Ward and Savagery are the key spells here.
With Defender's Ward, Molik Karn becomes DEF/ARM 15/20. That's tough enough (combined with some healing) to be a front-line beast, and go toe-to-toe with other heavies. Since he already has the offensive potential to scare heavies away, having the extra survivability, plus a high enough DEF for Intuition to be a noteworthy ability just makes him incredible.
Alternatively, Savagery can be used for the basic 'Molik Bullet/Missile'. Savagery adds +5 SPD when making a full advance, and hey- Fate Walker allows Molik Karn to make a full advance. That means Karn could advance (not charge!) into combat, and reach enemies that are 13" (6 SPD + 5 Savagery + 2 reach) away, Combo Strike (boosting to hit and for damage as needed due to Intuition), Side Step away 2", and retreat back using Fate Walker, ending your turn 15" (2 to start + 2 Side Step + 6 SPD + 5 Savagery) away from your target. He's mauled, you're safe, and it's a great way to take chunks out of your opponent's army without too too much risk. Obviously, you could also have him Side Step INTO the opponent's lines (multiple times if you don't Combo Strike) if that seems more important. So long as he's within 12" of Makeda (her control area) it's a scary thing to consider.
In short, this is a great pairing, helped even more by other choices.
Master Tormentor Morghoul: Abuse is always nice for getting the extra threat range and damage into Molik's activation (he then charges for a 13" threat and can get 13" away), but the d3 damage may not always be worth taking, since Karn can be VERY fragile. Admonition can be used to make up some of the difference in retreating distance though, keeping Molik Karn safe. Not a terrible match, but really Morghoul is decent with any beast. Morghoul gains some benefit from Fate Walker as well, but he doesn't have much fury to spare, usually. He has plenty of mobility though, so the additional movement can be pretty nasty.
Cyclops Savage: Molik Karn already has Future Sight, so gains nothing from having a Savage around. The Savage, however, moves from THR 8 to 10, which is a massive difference. No longer will leaving 1-2 fury on the Savage be a cause of tremendous stress.
Titan Cannoneer: Well, Diminish can be nice for keeping Molik Karn alive, but it's so unlikely to be useful that I wouldn't consider it much help. Molik's on a large base, so the Cannoneer will never fully cover him.
Titan Gladiator: A great inclusion with Molik Karn because of Rush. The extra speed is always helpful, given Karn's abilities, but the addition of Pathfinder just makes it so much better.
Paingiver Beast Handlers: As with any beast, they help a lot. Medicate will be nice, since a dead Molik Karn isn't killing anything, and Enrage is nice because Karn will heat even harder than before. Nothing surprising here.
Praetorian Swordsmen: No synergies.
Agonizer: The Agonizer does a better job than the Cannoneer for helping provide a little protection for Molik Karn due to the extra range. It's also a lot cheaper.
Ancestral Guardian: No synergies.
Molik Karn can be taken in either of the Makeda theme forces, and this will oftentimes be a great choice. It's also required for eMakeda's Tier 4 list (getting extra speed on the first turn) and counts for the Tier 4 requirement for pMakeda's (extending your deployment zone). You'll generally include him here anyway.
He can also be taken with Tyrant Xerxis' theme force, although including him won't provide much benefit aside from his normal abilities. This isn't a bad inclusion either- Xerxis can make Karn hit terrifyingly hard.
Overall, Molik Karn is a fantastic warbeast, although you shouldn't get into the habit of using him like other Skorne heavies. He does hit as hard as or harder than the other choices, but he is not so tough. He is capable of getting out of danger however, and this 'hit-and-run' style can really serve him well. He's not a great match for all of the warlocks though (as we'll see when I finally move on to other warlocks), and since he's pricey, he's not an auto-include to your lists. However, if you think you have a role for him, and the ability to support him some, you're going to love taking him.