This is a Dominion Master Yi build. I main Yi and this build is truly UNSTOPPABLE, even against Thornmail. By the time you get the first Phantom Dancer and begin the Infinity Edge you should be able to 1 v 2 squishies with no problem. There isn't much to Yi and with this build all you have to do is wack.

Attack Speed Glyphs to wack even faster.
Attack Damage Quints and Marks to wack even harder.
Armor Seals for good measure.

You should get Ghost so that you can speed across the map without having to waste you ult. Ghost also helps to chase down the other Yi in a Yi v Yi fight.

You should get Garrison on any champ in Dominion just because it allows you to turret dive so easily and without it you can't even get near their turrets.

Pros

Hits so hard
Crits so much
Runs so fast
Melts health
You don't lose health thanks to life steal
Ult doesn't even have a cooldown if you use it right
Good early game, Great late game
1 v 2 ability

Cons

Squishy, but with enough life steal you won't die
Armor counter you pretty hard
Ult only protects against slows
If you get CCed alot you will die