The biggest issue that game was probably how you built Sion. You rushed Tiamat, bought Frozen Heart, then bought Warmog's Armor before finishing Hydra. You had absolutely no bonus health until about 24 minutes into the game against an opponent with magic and true damage while playing the champion with essentially the lowest base health in the game.
You also had 21/9/0 masteries, pretty much all AD in runes (including some of your seals and glyphs, some flat and some scaling), and maxed W first.
Darius excels at crushing people who have no defenses or health and build offensive items. With how you built, by the time you actually had some tankiness he would aleady have snowballed anyways.
Though, the fact that he first killed you at 2:34 in the game indicates that you decided to try and fight him early. When you were extremely squishy. When he's at one of his stronger points. And after the first death you apparently decided to fight him again. On your first back, you only got some potions while he had a pickaxe and level advantage. He was much stronger, so you shouldn't have been trying to fight.
Next time, I would advise rushing Sunfire Cape, using 9/21/0 masteries, and maxing either Q or E first. W doesn't really need to be maxed, as the damage stays the same and it increases the mana cost. Additionally, you shouldn't be trying to fight Darius in this matchup, but should instead focusing on CS and maybe setting up a gank with your jungler.