It lasts for 5 seconds and has the heaviest slow outside of those 99% for .25 second slows. If this slow had a longer cooldown it wouldn't be as bad but at max ranked with 0 CDR it has a 11 seconds CD. Lets say nasus gets 40% cdr (which he normally gets at least 30%) the cooldown goes down to 6.6 seconds. Without any tenacity you will be not slowed for at least 1.6 seconds.
Jamaree4 votesBecause it does no damage, is a "soft" CC on top of being his only CC, on top of costing a third of his mana early game.
yukumari1 votesYea, but to make up for doing no damage, it's a pretty big attack speed slow, which is far from negligible when it comes to his combat power.
EndlessSorcerer4 votesI noticed you rushed Tiamat against Nasus. It's not a bad idea, but I find that you generally don't want to be pushing the lane against Nasus, since that just lets him farm under his tower safely and exposed you to ganks (which his Wither can easily set up). In addition, you didn't finish Ravenous Hydra for sustain, so you couldn't really heal in laning phase outside of potions or your Doran's Blade
It's something I've been experimenting with, but have you tried getting an early Brutalizer and Sunfire Cape/Randuin's Omen? The armor pen and CDR are nice, you can CS without pushing the wave, and it is cheaper than Tiamat so you can get some defenses earlier in the matchup. Before or after your first defensive item, you can build the Brutalizer into Black Cleaver for the armor reduction, which lets you hurt tanks no matter how much armor they stack. In addition, a little armor can go a long way against Nasus since he doesn't typically build any armor penetration and Randuin's Omen is great for chasing/kiting him due to it's active.
Also, I noticed that you started Doran's Blade against Nasus, which I wouldn't recommend unless you knew how to kill him early. I'd recommend starting with a Long Sword or Cloth Armor with health potions for the additional sustain in lane. While Nasus doesn't fight back very well early, minions can really hurt and you want to be able to sustain through it. If you insist on using Doran's Blade, I would highly recommend using 2-3 Lifesteal Quints with it.
Bit of an aside, but if you want to start Doran's Blade and have Teleport, you can do a jungle camp (I'd recommend the wolves) at level one, back to purchase potions, and teleport to lane. You'll miss most of the first wave, but you will be level 2 from the camp (so equal in experience or slightly ahead) and have an item advantage (Doran's Blade + 5 Health Potions + 1 Mana Potion).
Also, while you don't want to push against Nasus, you can spend quite a bit of time overextended in lane trying to zone him from CS. You didn't buy any wards all game and it looks like you only placed down your Trinket Ward twice according to the match history, which leaves you very vulnerable to ganks. Consider investing in a Pink Ward to place in the river bush or tribush and use the free wards provided by your trinket.
For masteries, Darius has enough innate damage so you typically want to run 9/21/0 masteries for durability.
For runes, Darius doesn't really benefit from the magic penetration marks you used, so I'd recommend using AD marks instead. Your quints are fine, though you might find lifesteal helpful if you want to start with Doran's Blade.
Finally, you maxed E second. I would highly recommend maxing W second in most situations because it greatly increases your sustained damage in fights, you tower pushing, and the uptime of you movespeed and attack speed slow.
Also, very critical advice for laning against Nasus: Do not push the wave unnecessarily. If you push the wave, Nasus can wait to farm under tower, where he is safe and you are overextended. This means that you want to avoid drawing minion aggro or dealing AoE damage to the minions early.
If possible, walk up to Nasus and see if he is stupid enough to autoattack you, which will make the wave start pushing towards your tower. If he doesn't attack you, you should try to last hit at the last minute possible or purposely miss a couple CS from the initial wave to make it push to your tower. Once it is in front of your tower, I'd suggest freezing the wave and zoning Nasus away; try and keep enough of his minions alive that they outnumber your minions, so your minions die without Nasus being able to reach them.
Kuroi861 votesBecause without it, kiting Nasus would be easier than kiting... well anyone.
Long Dong Wukong1 votesBecause if wither sucked he'd be getting 6 stacks per small minion and 12 per large minion to compensate, and his shitty slow would still let him Q you for 1900 damage once.
karruck1 votesI play nasus so ya he needs to be wary W,E,Q nasty combo at lvl 3 for any champ if i work it right ill have
by then working on 
the extra slows can be a gank easy
