The point of this build is to make an AD Sion that's sustainable away from home and hopefully doesn't use too much mana.

- Never attack without Enrage again. There really isn't any other option for the first item. Doran's Blade is hardly sufficient, so skip that.

- Great farming tool for the whole game. Occasional splashing in team fights. Recovery.

- Don't be slow. Get some crits. Attack faster. Save money.

- Postpone if you want, but you'll be getting a lot of health and it's natural to want to get this.

- As a life steal-dependent off-tank, you can't afford to be out commission for long. MRes never hurts, either. Sometimes I wish I had Boots of Swiftness, but then I'd have to put a Quicksilver Sash in there somewhere, and I don't know if I want to.

- Take that many more hits before you have to start LSing safer targets again. Get 20ish more attack damage via Impaler while you're at it. Can postpone.

- The Vampiric Scepter was more or less mandatory; may as well upgrade it.

- You got Zeal earlier. This is your last item. Everything in this item works with everything else.

Alternate items: Spirit Visage looked interesting, but either you're LSing like mad, or you're getting focused down to death. Damage reduction is probably more vital at this point. Force of Nature looks nice for similar reasons, and the MRes makes me take it even more seriously. Doesn't synergize with Atma's Impaler, but it sure beats getting maged to death.

You might need a Last Whisper before too long, in many games. If someone else is anti-tanking, then maybe you can let it slide. Most of the items aren't mandatory, so there are a lot of options.

might be tempting instead of Bloodthirster, but I find it a little redundant. Using your ult at the right time can heal your main tank or yourself or whomever else a lot.

Prioritize from right to left, pretty much. Q is just for stun. W is nice and should be used a lot, but we're not going with an AP build. E and R are going to kill things.

Pros:

+ Once you get Tiamat, you're gaining steam.
+ Lot of health
+ Decent damage output
+ Spamable damage reduction
+ Massive lifesteal, even without the ult
+ Sort of nimble, for a zombie
+ Pretty good anti-squishy (use stun)
+ Flexible build
+ Use Enrage at all times without suffering for it.

Cons:

- CC may very well be the death of you. Don't attempt to take on all tanking duties alone.
- Below 1/4 health, it's too dangerous to LS your way out of the hole you're in.
- It can be a little tough to farm the seed money for the build, but it could be a lot worse. Put up your shield, grab a last hit, and then retreat.