This build is only in use for the ultimative Jack in the box bush kill, i cant promise that this build will hold in teamfights without the use of "Jack in the box" But i will promise, that this build, will give you a great laugh, when your boxes rip appart even the toughest opponent.

Im going with Magic penetration to have a little more damage with your knife. You can go with full AP runes, but i personally observe that the Magic penetration runes will woke better in late game.

I go with the Deathfire Grasp as my first item, because when the enemy enters the bush, it's important that you'll have a fast nuke since my build dosn't offer a lot of defensive stats. After that i buy boots, which is a standard buy. After that i stack as much AP as possible, inclusive Lich Bane, agian for the knife throwing basis.
Banshee's Veil is a for longer life time when in team fights, also its passive works great for being bade for your "Jack in the box" trap.

Basicly i use Ignite for faster nuke and for the 10+ AP in early game
Flash i use agian, to provide a better bade for the trap.

I wanted to make a build, that owned who ever walked near the boxes, but also something that puts the knife into work. So i balance the knife and Box out as much as possible, and wait with the stealth skill.