This build is something I came up with to test a theory about cumulative magic true and physical damage on Master Yi. It scales with attack speed as the majority of your damage comes from the magic on hit effects and true damage from your e and jungle item. Late game this build will still do a fair amount of physical damage but not as much as some other builds.

Have fun with this build!

First we start out with skirmisher's sabre. This should be picked up on your first trip back to base so preferably after you hit 450 gold. Then you should finish the enchantment as devourer enchantment. This is going to be one of your main sources of damage if you stack it to around 80 to 100 stacks by about 35 mins. Along with your next buy the Wit's End you'll be dealing around 100 to 170 magic damage per hit at a rather high rate of speed late game.

Next you'll get statik shiv for the movespeed attack speed and crit and passive. With the passive hit every few seconds if it crits you'll be dealing about 400 on hit magic damage late game. Keep in mind this goes ontop of your physical damage and true damage and criticals.

Next you should get Trinity Force start with the zeal for movespeed and more attack speed and critical. 1 out of about every 3 hits should crit now. Next get the phage or if you're doing well but need more on hit damage get the sheen.

Then pick up your Blade of the Ruined King for some life steal and the awesome on hit effect. This item will help you shred tanks even faster particularly those stacking health. It also offers an amazing active effect which slows allowing you to catch quite a few kills mid to late game. I sometimes get the vamp scepter or the cutlass earlier in the build if i need sustain or need to slow enemies usually right after the last whisper.

Lastly we will round out the build with Ravenous Hydra, this gives a great ad boost active effect regen lifesteal and large amounts of aoe allowing you to cause havok in team fights.

This build will end up giving you max as even without ulti aroudn 415 movespeed with dragon buff, around 300 ad, 30% crit chance, about 200 to 400 magic damage on hit late game depending on the statik shiv proc, a metric ton of true damage due to your e and skirmishers and some bonus health and magic resist. It could be called Yi the trinity force as it offers him a bit of everything.