Mercs are lower than tabbis because tabbis can be rushed as spike items against a lot of champs. With mercs you shouldn't really rush them except like kennen and gragas
Fon is an excellent item for darius but i don't like it that much as second item. Buying either a negatron or null mantle and then going dmp or dd feels better than completing fon right away if they are 1-2 ap
Mercurial scimitar is really slept on. Insane item against cc, movement speed is great and magic resistance is really appreciated
I still think stride is the worst out of the 4 most built first items
Hullbreaker is good against random bs enchanters on top like zilean or ivern coupled with demolish. And also ok if you completely doomed your lane+you are tilted and can only split push
Jak'sho is a great item, but usually too expensive to fit anywhere in the build.
Sundered is also ok,but i never build this item. There are almost always better options but i know some people like it and it's decent on aram .
Maw and hexdrinker is also really slept on. Really good as rush item vs morde and gwen. The shield scales with ad, so you get insane shields even early on,omnivamp is also nice. Be mindful when you buy it though,it locks you out of steraks
Only buy spirit visage when you have a strong enchanter in your team and need mr, otherwise there are better options
Mortal reminder is the best anti heal item on darius. Chempunk is ass and only buy thornmail if you bought bramble in lane and even then finish it last
You shouldn't really be the one to buy serpents fang. But i guess the stats are ok
Eternal hunger is only good into heavy melee comps. Otherwise never buy
GA good mostly when they have a lot of ad and you need to protect a big shutdown
Rookern is outclassed by other mr items. If they are 4-5 ap and you have a decent amount of bonus health then it's good to buy
Ioanian boots also fall into the "okish but always better options" , summoner spell haste is really good on darius but never worth sacrificing mercs or tabbis for it. Upgraded ionian boots used to be sleeper good but now they are only available on mid and they got nerfed so meh.