So the premise of this build is to gain as much HP as possible as quickly as possible.
Clearly in 3v3 maps you shadow dash can let you escape almost every death, as you can slide through most terrain, and the terrain you can't slide though, you can flash through. This will allow you to keep most, if not all, of the stacks you get on your leviathan. Flash and shadow dash also let you get your ignite on most people who are close to escaping, as you can cover a large amount of ground very quickly.
Your ult should be saved for saving your team, which can sometimes be coupled with getting kills. All around try to err on the side of support and have the least amount of deaths, so that you can later tank the turrets.
The health potion can be sold later if not used, yet if you play correctly, it should be used after the initial gank at bottom. When you back for the first time, buy either a leviathan or a heart of gold. Their are interchangable. Keep in mind that the heart of gold will let you buy your boots and leviathan faster, but getting the leviathan will allow you to get more stacks quickly. Warmogs adds hp, and if you get to the point at which you can get a guardian angel, you should have at least 4k hp, which means you will be a 6.5k hp tank counting the respawn at 40% that guardian angel gives you.
--Reason for Runes and Skills--
Vorpal blade gives you the ability to heal, which allows you to stay in lanes until you must buy. Obviously get your ult. Feint does allow you to block damage, but it does end up taking your energy too fast in the long run to prove useful until it can block about 150 damage, but that is around levels 3 and 4, which makes it useless to buy early on.
A full sheet of AP runes and magic penetration help you with harassment early game, as you get an additional 20-25 damage on your vorpal blade, along with 10 magic penetration.
Masteries are open to debate. I get cd reduction and health bonuses so that you can constantly blade, and so that your ult is able to help very frequently.
comments are welcome and encouraged.