Dragon of Greed

Fafnir is a powerhouse with tools for every situation and can dish out damage even with pure defensive builds. Since he's a Guardian first and foremost, you will want to build him with survival in mind, using items that can balance defense and offense at the same time.

His passive is great, and this build is made to save you gold by not needing consumables as much as other characters, so you don't need to spend all your gold at once and can save some to keep his passive working.

<Watcher's Gift>
This item helps you gain even more gold and gives you an HP boost, what's not to love about it.

<Shoes of Focus>
The Mana boost and Cooldown are more important then more power and penetration at this point in the game, since you'll want to stay out in fights for longer periods of time then most other characters.

<Warlock's Sash>
At the full 100 stacks, this item gives you 110 power and 700 HP so why wouldn't you want this for a guardian? Get this early so you can build up the stacks, and with Fafnir's passive, you won't behind in gold, so take advantage of it.

<Hide of the Urchin>
This is harder to build stacks but also faster since you only need assists. In total you'll get 55 points in both protection stats. This along with your passive gives you great protections for little effort.

<Pythagorem's Piece>
This boost to power helps your sustain with good lifesteal and cooldown, in addition to it's aura for your teammates since guardians are team fighters, it's helps a lot in arena, joust and clash.

<Bancroft's Talon>
This offers more power for less lifesteal and no cooldown. I would only use this in conquest unless you have a mage with you in long lane like Aphrodite or as your jungler like Ao Kuang.

<Mail of Renewal>
400 HP with cooldown and you gain 15% of your HP and Mana back when you get kills, assists or objectives. This is good for more aggressive players.

<Stone of Gaia>
Same amount of HP, but now you just gain HP every 5 seconds, so you don't have to be as aggressive and pace yourself, only downside is no cooldown.

<Ethereal Staff>
In late game, replace your starting item with this for a surprising 128.625 power boost in addition to 400 HP and 300 Mana, no other defensive item with give you such a big power boost.