This guide was made specifically for a friend that is somewhat new to Dota. It is not meant to be a very great guide for anyone else. If you choose to continue looking keep that in mind.
As you already know, Legion is a character that excels in survival ability and being able to pretty literally get infinite potential damage. Early game you can be an immovable force from lane making it easy to practice last hitting which will really give you the edge. Depending on the lane, they may try to pressure you pretty hard but you healing items plus the two healing moves should allow you to sustain most lanes. Legion once dueled relies on her damage as well as her team to get the duels. Early game this is fine, however late game you should be able to dish out most of the damage on your own.
Overall I tried to make the build as simple as possible maximizing items you don't need to click on so you have less to focus on. Just try and get the general vibe of what you're trying to do before you add in the extra items that I put under situational.
Early Game Items
The reason for these items is to give you the easiest farm possible and account for the idea of hard harass. Normally, a tango or two will allow you with your skill build, to lane perfectly and keep you nice and healthy for your farm. This isn't always the case as some lanes put a lot of pressure and try to deny you. The Qualling is to make last hitting easier. The Shield is to brush of attacks from both creeps and enemies. Tangos and healing salve are just in case **** happens items.
Used for getting into fights and out of fights. It is a key item to make sure legion is able to pop on a duel and have plenty of damage and attack speed to kill the opponent.
Sange and Yasha
Sange and Yasha gives you a massive boost in chasing ability increasing your movement speed as well as giving you a slow. The extra strength and damage as well as attack speed boost will be excellent for giving you a little push into late game but give you a bit more mobility as well.
Basic boots that some people would say you need to exchange for phase boots due to the extra movement speed and damage. I personally like being a bit more tankier and attacking faster and it proves useful in duels.
This item gives a lot of armor reduction to the enemy as a debuff. It is amazing at taking towers and helping you do more damage piercing right through their health. It helps you end duels quickly and melt right through the enemy and base. Can be taken up as a third or fourth item typically.
This item makes you and your allies tankier, makes you attack faster, and reduces the enemy's armor in an aura around you. It can be useful for stacking with desolator to give you a bit more of an edge on enemies that deal a lot of basic attack damage.
Heart of Tarrasque
This is for if you are getting hit hard. This makes you hit and have the ability to get hit and regenerate your health in the back lines. Makes your health spike and your survivability go way up if you don't have many ways of dealing with enemy CC (Ex Faceless Void's Ultimate)
An Item everyone and their alcoholic mother will tell you that you need to be a good legion player. This is a SITUATIONAL item and not 100% needed in every game. This is an item that will deal an enemies' damage back at them. You are supposed to pop this before you duel to be able to capitalize on the enemy's own damage while somewhat safeguarding yourself from other enemies that may try to hurt you. This is a great item but is in the back and must be gotten AFTER your ability to move around through blink dagger or shadow blade. If you rush it you are asking to lose the game. I low key have a burning passion of hatred for every legion player I see rush this dumb ****.
It is best to get when an enemy that is a carry is reasonably fed or is a glass cannon.
Good people to get this against would be PA, Bloodseeker, Troll Warlord, Clinkz, etc. Anyone with low health and massive damage.
A substitute for shadowblade or a lovely addition with it.
This gives legion a lot more mobility and will allow her to really get in and out of fights for those quick duels. You get the choice of blinking in on an enemy and fighting them (typically needs vision) or being able to be stealthy and fight someone quickly then blink away before enemies can pounce on you. This can be a deadly combo but you're exchanging being tanky and dealing more damage for this.
Most legion players go for this route and stalk the jungle treeline for enemies to blink on. In my personal experience, it is much better to have the stats from shadowblade along with the movement speed, and being able to go into places you don't normally have vision. If you have a few good buddies to play with, it would probably be a lot easier to run this but it does allow you to hit the enemies backlines in an instant. It is also slightly cheaper than shadowblade by about 550 gold but doesn't have anything it builds from so it could be more of a risk.
Aghs makes the duel longer allowing you to capitalize on the extra damage a bit more but the more important part about this is it makes the duelists the only ones able to hurt the other person. Stuns and CC still go through but the even more beautiful thing about this is the fact you now have magic immunity in the duel.(Note some **** goes through even magic immunity but most damage from these things does not.)
This item can be a double edged sword if you need teammates to help you kill someone massive. On the other hand, if you are already miles ahead it gives you the ability to lock down any one character without worrying about any annoying CC or damage ultimates. However, the second this duel ends... Good luck.
Black King Bar
A go to if you are getting constantly *** blasted with stuns and other annoying little bits with silences. This debuffs most bad status effects on cast and gives you magic immunity for a small window of time. This is only really necessary if the enemy is organized and trying to shut you down and you already have some damage items to rely on. Is not needed as a first or second item in most games.
Armlet of Mordiggan
An amazing item for toggling and getting used to using items in the game. This can be a life saver if used correctly and your best friend on some characters. For Legion, I've seen some people toss this on because you can activate it before the duel to give yourself a nice boost in damage and health during the fight.
A problem with this is item is with it's active, it constantly drains your health and that will stack up over time if you let it. Make sure you have it turned off when you are not using it.
Toggling this item is turning it on and off to give yourself a boost of health. If you are let's say at one hp and you have a projectile coming at you at a slow pace, you can activate this item and let the projectile hit you. It will allow you to live early if you are able to use this effectively but takes a lot of practice. You CANNOT DIE from the hp drain.
Satanic is an item that makes you rather tanky and adds a bit of life steal. If you find that after a bit of stuns you survive with about half hp, Satanic is something you may want to pick up. The active is what makes this item so expensive because with a single proc from your E with it also active you can go from 1% hp to 100% hp. It makes it a good buy but still a bit situational if you are getting hit hard with CC. It also packs a bit of damage behind it with a little bit of hp.
If you are finding people instantly hit shadowblade, have a Linkens, or silence to stop you from locking them down, Nullifier is a great item to help with this. This not only mutes them from using any items, it also slows their attacks by 100% when they are hit for the duration. It can be used to hardcore shut down enemies or make the little rats easier to catch and duel. Should be used with only one enemy carry is doing really well to shut them down without having to worry much about the enemy team.
Another single target item that works amazingly well with Nullifier. Can be combo'd to completely silence an enemy from using items or abilities and you will be doing MASSIVE damage to them without them being able to fight back much at all. It also applies the ability to completely pierce evasion for all allies to hit them, crit them with a 100% chance, and does 30% of the damage dealt at the end of the duration. It can obliterate an enemy and completely take them out of a fight. Used in situations where a single target has annoying *** spells.
Because Bristleback doesn't actually have to hit you in a lane to deal his massive damage, he can put himself between you and the creeps without needing to really worry about your healing abilities. If you let him get too many shots in, he will probably run you out of lane and put you behind heavily on farm. If this happens, try to get your laning partner to pull the wave, assuming they are actually playing a support. Fight under our just out of tower range (whichever you feel is safer at the moment) and be ready for some aggression because they will be thirsty for some payback after the first pull typically. Late game he isn't as much as an issue but is typically not someone you want to duel due to being very tanky.
Crystal Maiden, Lion, Shadow Shaman
These three supports in particular can be real pains in the *** not only to lane against but to have on the enemy team. Because you're looking for fast duels and will be unable to help yourself by casting spells or moving you will have to be a bit more wary when playing around these characters.
In lane, Crystal Maiden can frostbite you to hold you down for another enemy to stun you. You can easily counter this by using your heal on yourself to negate the effect of not being able to attack and rooted.
Lion and Shadow Shaman can be a problem in a duel and out. Their hexes are strong making it to where you can barely move at all and can't use abilities. They also have stuns which can be very long and very annoying without a partner to knock them off you.
Troll warlord is one of the characters you should duel only if they are pretty low or very underfed. The reason for this being is when he switches to being melee, he can be extremely annoying with his bash chance. He has the ability to keep you stun locked based on his luck and can be a real pain in the *** for what should be an easy duel.
Enchantress isn't a target that is exactly painful to go up against unless she can see you. Her passive Q will make you have an insanely low attack speed against her. That and if she is able to pop her healing faeries you may have quite a long duel on your hands that will just end up in you being a target for other enemies for a few seconds.
Silver Edge is a solution to this as it allows you to temporarily disable this passive.
Never have idle hands.
As a carry there are only two acceptable times for your character to be idle:
If you are positioning or waiting for a nice fight.
The start of the game before the creeps spawn.
Other than that you should always be on the move and looking for money, xp, and duel damage.