Basically what this build allows you to do is to stay alive while still doing damage. The typical items hextech gunblade and Zhonya's Hourglass are still built. Hextech Gunblade allows you to sustain, giving both AD and AP, Life steal and Spell Vamp. The active on this item allows you to assassinate slippery enemies. The attack speed plus extra on-hit damage (Spellblade passive on triforce, percent of ability power on Nashor's Tooth) helps you push out lane and decimate towers. Remember that Akali's passive gives a certain amount of magic damage on basic attacks based on ability power. The attack speed also allows you to utilize that life steal from Hextech Gunblade. The Zhonya's gives you a large AP boost and the active allows you to stay alive when in great danger and gives you time to think about an escape. Finally, the Void Staff gives ability power and percent magic penetration. This is helpful with the magic damage that you deal with your abilities and extra on-hit magic damage.

Spirit Visage gives you a health boost, magic resistance, cool down reduction, and health regeneration per second. Adding to this, you get 20% increase in self-healing, which synergies with the second part of her passive that gives bonus spell vamp based on bonus attack damage as well as the spell vamp and life steal on Hextech Gunblade. Having this in your pocket gives you extra sustain plus the health boost and magic resistance helps you out when in team fights.
If the enemy team has little to no AP and is a heavy AD composition, you might consider replacing the Spirit Visage with something like Randuin's Omen or Sunfire Cape. Although this may not give you the self-heal increase, cool down reduction, and health regeneration, it does give you a health boost and extra armor. Again this would help you stay alive longer in long team fights and when you get caught out by enemies.

This may not be the huge damage dealing assassin of the mid lane we all no and love. That's the point. This build allows for you to be an off tank, tower pushing, team fighting annoyance. She may not do as much damage with this build, but she can still burst down an enemy, if built like this during a team fight she could kill more than one and still have enough health to safely push the lane or even take a tower. The attack speed helps you chunk towers, and with the movement speed boost passive with TriForce you can keep up with enemies who are trying to escape even after you use all of you ult stacks. If used correctly, you will not be underestimated by the enemies when you confront them. Enjoy playing this off tank damage dealing assassin up there in the top lane!