First, to note, I am not a very pro dota player, and this guide is merely based on what I feel worked. There are probably better guides out there, but this is the build that I am most comfortable with. Riki was the first hero that I started playing when I began dota 2. This is an amazing hero, with a potential beyond imagination. If it is developed properly, it can be an ultimate hero. The trademark of Riki is definitely its invisibility. This is a skill that can irritate your opponents like mad. When playing this hero, it is important to lane with a hero that is best a ranged; this would ensure that s/he would be able to discourage enemy melee heroes from going in to attack you. After level six, you can either stay in your level to farm for gold, or you can start ganking the enemy, bringing about endless chaos.
The skills that Riki has are diverse and all for a specific function.
The first skill is Smoke screen that is very useful is team fights or ganking. This skill provides a 25% slow and a 70% miss chance at level 4. This meaning that the enemy would practically always miss and would not hit you when he is in smoke. Leveling this at early levels would not be useful as your damage would still be pretty low and cannot kill the strength heroes fast enough. This skill also renders the buying of butterfly useless since smoke screen provides the evasion you need.
The second skill is blink strike which allows you to chase enemies. At the start, do not hesitate to use this skill to irritate the opponents and they would likely run away when you appear behind them. At mid or late-game, the damage that this skill provides would be rather insignificant (a meager 120), this skill would thus be used mainly for the purpose of chasing enemy heroes as there is a low cool down, you would be able to use it frequently. However, I would prefer to cap it at level 1 at the start of the game and max it later one.
The third skill is backstab and can be said to be the most important skill in terms of boosting you attack. Without this skill, Riki would be really weak. Be sure to max out this skill in early levels. This would ensure that your damage output becomes crazy.
The fourth skill is invisibility, is the trademark Riki skill and can be the most annoying skill in the game for your opponents. This skill is not only good for ganking, but also great when you want to escape. Due to the short fade time of the skill at level 4, it is practically impossible to kill you. However, what is important when playing Riki, is to keep a lookout for which enemy hero has bought a gem of true sight. This item is deadly against you. It would strip you off all advantages of invisibility, making you easy to gank and kill.
The combo for this hero when you target an enemy hero is to first go behind him when you are invisible, then throw a smoke bomb. Ensure that he is at the center of the smoke so as to ensure that he would take the longest time to walk out of the area of the smoke bomb and you are given the greatest amount of time to attack him. After he exits the smoke bomb, use blink strike to go behind him and continue chasing him. He would practically be dead once he is spotted alone and you target him. This is however; only possible when you have a good build that ensures high damage output.
Now I will explain the rationale behind the item build in late game.
Daedalus would provide critical hits that can greatly boost your attack when the enemy tries to take you face on and you cannot get behind him to attack him. This therefore is important in late game if you want to keep your kills up.
Mask of Madness is another item that can boost your kills greatly. The life steal that it provides is important and more importantly is the active skill (+100 attack speed and 20% movement speed; however you will take 30% damage). The additional attack speed can be deadly when you initiate a fight with another hero and the 20% movement speed allows for quick chasing. This skill should only be used when you want to finish off an already dying hero. The 30% additional damage intake would mean that you should not use it when you are involved in a team fight where you are outnumbered and the enemy heroes are targeting you. However, when doing a 1 on 1 this can be deadly.
Urn of Shadows is an optional item that you should buy if not one else on the team intends to buy one. This would ensure that after you run away with low health, you can heal yourself.
Radiance is an important item as the burn damage can be extremely useful. With the burn damage and you staying invisible, you can irritate enemy heroes and force them to retreat. However, do keep in note that you should not do any attack when using radiance so as to not reveal yourself to the enemy. Similarly, do not use this near towers as you would be revealed. However the down side of radiance is that the opponents know that you are there and would keep their guards up. Only use this when you are preparing for a team battle.
Shiva Guard. This is an important item for you to plan your escape route when you are dying and outnumbered. First, use a smoke bomb, and then activate Shivas guard to slow the opponents movement like ****, allowing you to escape quickly.
Phase boots is the best form of the boots that Riki should have since the main focus of Riki is maximize his damage output and also ensure that he is quick and agile for escape, flicking in and out of sight. This is not only a good item for escape, but also a great item for chasing down enemy heroes. However, it is important to note that when you use it, you become visible, so do keep a lookout when you use it and avoid being ganked.
The hero that threatens Riki the most is bloodseeker and when you are targeted by him and incidentally he is not a noob, it would be wise to run. If you are inflicted by his ulti, your best hopes lie within dealing the most damage to him before you die.
To sum it all up I would like to provide a few important tips. There are a few important points to note when you use Riki. First of all it is to communicate well with you teammates so as to allow efficient ganking. Secondly would be to constantly keep track of what enemies are buying. Thirdly is to avoid AoE skills as they can affect you even when you are invisible. Fourthly, it is to allow yourself to be seen at the right time. Fourthly and lastly, if the enemy is not running away and decides to take you face on in early mid game or even late mid game, it is a sign for you to consider running, if not you most probably would end up dead, especially if the opponent has a stun skill and AoE skill.