Tiamat is one of my favorite items in League of Legends.
Because it provides wave clear and damage for combos.

This item gives you mobility and lethality at the same time.

This one gives you lethality which is essential for almost every AD assassin.
gives you more health but if you don't like this item you can exchange it for .

Passive works like basically by every spell you hit, it gives opponent one stack of passive, you can activate bleeding when opponent has three of these stacks by auto attacking opponent.

Q is leap / melee burst spell used to get close to your target.
Also this spell resets your auto attack.
Great last hitting spell.

Rake has a very good wave clear.
If you hit opponent with it and it returns and hit opponent again.
Opponent will have two stacks of you only have to hit opponent with .
To complete combo.

Your E allows you to jump over terrain or structure.
Your vaulting speed depends on your movement speed.

By casting Shadow Assault you become invisible for the time your blades are in air.
It also gives you small burst of movement speed.
If you attack any enemy your blades will travel to enemy's location.

Basic combos are:
Combo #1:
+
Combo #2:
+ + + Auto attack
Combo #2 with Tiamat
+ + +

Basically you are focusing the most squishy enemy you can usually it's ADC.
If you are in middle of teamfight and you don't know what's happening use Shadow Assault to observe your target or tactically escape.