I jumped on the Rengar Freelo Train not too long ago, and I'm sure there are enough posts about freelo Rengar. The takeaway that's relevant here, though is a quote from Ryan Choi, the Rengod: "If you're not winning lane, it's because you don't have enough Doran's Blades". Thing is, this isn't a Rengar-exclusive phenomenon. Back when Riven was huge in Season 3, it was common knowledge. Build Doran's until you're shitstomping your opponent, then worry about real items. Well, it still works. In fact, it works for *most* top laners, if their goal is to do tons of damage in the midgame and not to soak up damage.
Enter Darius. Traditionally, he's had a couple of glaring issues. First of all, he "needs" to build full tank. Why? Because his engage is "just walk into the fight", so he's gonna take a lot of damage in the process. Fortunately, he has high base stats, so he can still do damage without building it. The unfortunate side effect of that, though, is that people overlook his ratios, thinking that "Darius builds tank" because "Darius isn't designed to do damage". Bull-fucking-shit he's not. Each passive stack has a 30% bonus AD ratio. Q has a 105% bonus AD ratio (if you don't suck). W scales up to 100% AD bonus damage. The Dunkening can shit out a 150%-AD bonus ratio. Also, Darius is a champ that autoattacks. A lot. Pull -> Q (70% AD) -> W (120-200% AD) -> AA (100% AD) -> Tiamat (100% AD) -> AA (100% AD) -> Q (105% AD) -> R (150% AD) -> bleed (150% AD) is what a normal full combo looks like, and for each point of AD you buy, you're getting 8.95 to 9.75 more damage on a full rotation. Yes, each Doran's Blade gives your burst about 70 extra damage at level 6. Yes, Ravenous Hydra gives your burst about 750 pants-shitting extra damage at level 13. Darius' AD ratios do not fuck around.
So instead of doing a sunfire rush or whatever, build Doran's Blades until you're dumpstering your opponent. And then build Tiamat, and Brutalizer, and Last Whisper. These items put the "Dunk" in "Dunk Marathon". Your last core item is Boots of Mobility with Alacrity, and you'll upgrade Brutalizer into Ghostblade (not Black Cleaver) and that's the "Marathon" part. You're going to run around the map so fast and dumpster people so fast when you find them. They can see you coming and flash away; doesn't matter. Too fast, too furious. This also solves the "Darius can't engage" problem. Yes, you will engage by running into them and fucking everyone up. They're grouped too hard for you to go balls deep? Fuck up their turrets. You have a lot of AD and pen and turrets don't like that at all. Someone comes to stop you? Give them tickets to the Dunk Show.
Runes: 15 AD, 9 flat armor, and whatever blues you're feeling. I run 5% flat CDR and 5% scaling CDR because the rune page is for Riven and she hits 40% with those runes, 5% from masteries, CDR boots, and Brutalizer, but you're not buying CDR boots. Dunk MARATHON, remember?
Masteries: 21/9/0 or 9/21/0 both work. I'm very tempted to take 3 points in utility so I can run even faster.
Summoners: Ghost (Why? Dunk MARATHON. MAAAAAARAAAAATHOOOOOOOONNN!) and Ignite. If your lane opponent takes Teleport or some shit, it's a free snowball because they sure as shit can't scrap you without a combat summoner spell.
Item build: Are you winning lane? If yes, buy Tiamat, LW, Brutalizer, and Mobis (MARATHON!). If not, buy more Doran's blades (try for 3 or 4). Done with all of that? Grab Randuin's, Sunfire, Banshee's, or Spirit Visage depending on what the other team's damage is. Upgrade Brut to Ghostblade when you need more dunk and/or marathon.
If you decide to join the marathon, post results below. Get out there and wreck face.