Ok, I have some suggestions for what to build and how to beat him. Tahm is definitely stronger against juggs like darius than he should be, and can often face tank beat a darius early on, but nonetheless, you should be able to beat him handily in trades after a certain point.
I usually go grasp for the easy procs against tahm and the fact that I just don't enjoy the heavily nerfed conqueror very much anymore. That being said, some think grasp is bad on darius, so it's really up to you.
I start by buying ruby crystal and a long sword on first back, since boots and a sword won't do a lot of good (though boots and hp might be good) and you probably won't have gotten lucky enough to have actually killed him by that time, so you probably won't have enough for phage.
By second back, I tend to finish phage and start building mr. It's paramount to build a spectre's cowl and at least get base boots at this point. It's NOT about getting more damage yet since A: you need cleaver over Tri in this matchup and you won't have enough gold to finish cleaver by next back, unless kench is bad and fed you a couple kills B: surviving longer in fights as well as sustain between fights/damage mitigation is more important that damage at this point.
Why? because kench has plenty of damage at this point, but easily out tanks you unless he plays terribly, and on top of it, he has means to dodge your second or third q (when you're low and it really matters) simply by eating you at that point, and also delaying your stacks. You will last longer and actually do more damage by going hp and mr early on after phage. Also, kench will likely land some Qs from a distance but be unable to force a fight, in which case the regen from spectres cowl is nice.
then after that, try to finish either cleaver or spirit visage. If you're winning trade just enough and if kench is a little overly aggressive, go cleaver. If you're losing trades, go visage. Also, if you determine to go merc treads (a good idea if they have an ap mid and someone like rammus jg, as in your scenario, and if their adc isn't massively fed), get them either just before or just after finishing spirit OR cleaver, but don't wait too long. because unless you decide to go tabi (in which case, hold off until after lane unless they have a high ad jg), you're gonna really want that extra tenacity and mr.
The way to play with this early game build is to be rather passive early on. Try not to push lane too much with Q, as we all know kench has plenty of cc to keep you in place for a gank, and if you are too close to tower, he can spit you under. Just stay behind your minions and use W to last hit as much as possible. Using Q to poke him early, which usually pushes the lane, isn't super beneficial since he is beefy as heck, and usually sustains pretty well. Instead, I like to farm and get grasp procs on kench when he goes to melee. You'll take some damage, but you should be able to survive trades until you get cleaver and/or visage, in which case you will start to win the face tanking fights. Of course, if he does fight you when you auto for grasp procs, by all means go ham on him, since you likely aren't going to run away without taking some decent damage. If you push the way a little because of that, just be sure to either shove it in hard to reset, or allow it to push back.
Obviously overall though, the key is stick behind your own minions. If you're not afraid of pushing the wave too much at certain points, by all means poke him with Q from behind minions. Essentially, kench is stronger early in a brainless smashing contest, but darius should never lose so badly that he can't escape when it turns bad if he has at least 75% hp at the start of the fight. If you're doing well and use this build, you should be smashing kench pretty easily by about level 8, and maybe even getting double kills if you stack up on kench quickly just before enemy jungle approaches.
is rather effective.
His W does a good bunch of %hp so stacking HP is not the best idea.
He doesn't break any defense at all.
He doesn't heal of minions, if he heals thats his E, which converts damage he takes into grey health, and then he can either use that for his shield or some of it regens back as health after a while.
