How To Build Jarvan IV - Lethality

Bruiser versus Lethality

In order to single-handedly carry games with Jarvan IV - you need to build him properly. Jarvan IV has one of the best AD scalings in the game, and combining that with his natural % armor pen from Q and AoE damage from R - he is an assassin in disguise. Therefore, building him lethality with some "fake tank" items is by far one of the best options.

Runes

Because we're opting for Lethality, going for the usual Conqueror setup is just not optimal. You're playing to one-shot the enemy target, not to play delayed teamfights with multiple EQ rotations and sustained damage. And what better rune for one-shotting than Electrocute? Combine it with Sudden Impact for extra Lethality on the EQ combo, Grisly Mementos or Sixth Sense for vision control, allowing you to sneak up on enemies, and Treasure Hunter combined with Cashback and Triple Tonic in secondary runes for snowballing the game. Small shards will of course be double Adaptive Force and Scaling Health.

Jungle Pet

Going for the green pet is in my opinion just the best; you already have enough damage not to need the red pet, and you will move around the map using EQ, so movement speed from blue smite is not as valuable, while the green pet allows you to still be a "tank" while building full damage.

Core Items

Profane Hydra - By far the best item for Jarvan IV and one that you should almost always build first. It allows you an insanely fast clear, which you need as you're playing an assassin and a carry who needs as much gold as possible. The second thing is that you can use the active with any of your abilities: Q, EQ, R, or just AA and Profane, allowing you to squeeze in additional damage during your burst.

Axiom Arc - You're usually looking to pick Jarvan IV into immobile champions, and you're also building him full lethality, making his ultimate a very important spell to have up. Dropping its cooldown on a kill allows you to have more kills in the early game, and later on, use your R on the first target, get 2 assists, and use another R on another target. Not to mention that all the base stats from Axiom suit you. The only Lethality alternative could be Bastionbreaker, in case you have really bad R targets, but then you're playing the wrong draft for Jarvan IV.

Death's Dance - The absolute king of all bruiser items. This item single-handedly makes you a tank, while providing you with additional AD and AH. The only exception for skipping this item as your third option is if you need it earlier (vs. Rengar, for example) or if the enemy team is very magic damage heavy and Maw has higher value. Other than that, go for this item third every single game.

Boots - Usually I don't finish boots before my first two items, often not before my third item, looking only to sit on Tier 1 boots. That's why going Magical Footwear instead of Cashback can also make a lot of sense, but I personally just prefer Cashback because you're getting your first 2-3 items ahead of the curve and I think it just snowballs better. Regardless, the reason you're not finishing boots early is because then you would spend gold on something that is not damage, which you need as you're playing to one-shot opponents. The extra MS feels nice, but Jarvan IV has a very high reach with his EQ and R combo, can often find gank angles from fog of war, dodging vision, and you will use EQ to jump over walls while doing camps, so the MS, even though it is nice, doesn't feel as necessary. But once you get to the point where finishing boots is fine, you will most likely look for defensive ones, like Steelcaps if there are a lot of champions that auto-attack on the enemy team, or Mercury's Treads if there is a lot of CC on the opposing team. If neither is the case, you can stay on Tier 1 boots until the last item. Oftentimes on 2 or 3 items, if you're recalling before an important objective or play happening and you find yourself with 600g, finishing Lucidity boots isn't a bad option; it's very cheap so it doesn't hurt your snowball as much, but it can make a difference in a teamfight.

Situational Items

Maw of Malmortius - This item, even though put in the situational tier, is built in most games, as long as the enemy team has magic damage that you need to survive. It's the only item that gives attack damage and magic resist, so there aren't any other options to look at.

Endless Hunger - Even though we're playing to one-shot, this item can find its purpose later on during the game, as it will give you 30-40 AH alone, making your EQ cooldown very, very low. Combined with Death's Dance, there is definitely some pop-off potential with this item. In team comps that have a lot of CC, it also has additional value.

Guardian Angel - Very good item if your job is to go in and die. Oftentimes, if the game goes long enough, the enemy team has a lot of tools to survive burst, or they are just permanently grouping so it's hard to isolate one person. In games like that, going in and just pressing R with tons of AD on multiple targets will get them low enough for your team to clean up. I like to build it with Karthus, Katarina, Samira, or Jinx on my team. And of course, it's very good against champions that need a reset in order to jumpstart a teamfight, so against Viego, Jinx, Katarina. Keep in mind that in the late game, after reviving, you will often either have EQ up, or Q up with a flag on the ground, so you can immediately play the game and look to continue fighting. It's also a decent item if you're so ahead on the map that you're finishing your build and holding a 1k gold shutdown, allowing you to play extremely aggressively.

Serpent's Fang - This item is a must against champions like Karma and Ivern, but please don't ever build it as a first item; even as a second, it's often not needed, depending on how much shielding they have on the team and if the enemies are maxing their shields. Usually, I build it as a third item. It's also good vs. champions like Shen, other enchanters like Lulu and Janna, and also duelists like Yone and Yasuo who also like to build Shieldbow.