Number Crunching Fafnir

This is a buffer/tank build. It focuses on CDR, protections, and items that will proc from coerce. This build tops out at 251 physical protections (lvl 20 and full passive), 187 magical protections (lvl 20 and full passive), 35% CDR, and 30% CCR. The glaring weak point in this build is its relatively low magical protections; which can be compensated for, if required, by replacing void stone for pestilence (bumping up max magical protections to 217).

Endless Greed

Bonus gold + bonus protection = lotso love!
This passive allows you to pick up watchers gift, let your ADC take all of the last hits and you will still be rolling in the gold. The protections are a nice bonus, and their are competing theories on how to treat this mechanic. Do you make sure you are always sporting 500+ gold for that sweet sweet protection bonus? Even if that gold means you could finish up some rad new item. Or do you just spend everything you can get your awesome items faster than any other support could dream of accomplishing, and treat the protections bonus as an after thought? I personally go for option number 2, but to each his/her own.

Cursed Strength

This is your only hard CC outside of your ULT. Its kind of pathetic IMO, but it does help in a pinch. The damage is acceptable and the range is nice, but this ability overall is nothing special. I personally use the hammer for a decent lane clear, that might possibly catch a guy in a stun. While in dragon mode this ability also comes paired with a pretty baller protection shred (40%), making it a mean ability if your ADC happens to be standing next to you. Second ability I max out for the damage.

Coerce

This ability is what makes this build work. It applies an attack speed buff to a single ally. But not only that it applies a 25% damage bonus that scales off of your allies damage, but behaves like the damage is coming from one of your abilities. This means you can pile up some on-ability items and have some fun with them. Coerce your hunter and watch as your Gem of Isolation puts a 25% slow on the baddies with every one of the hunter's AAs. Hence the Soul Reaver, Spear of the Magus, and Divine Ruin. All of these will proc from an attack by any of your buffed allies. This abilities self heal also gives Fafnir some decent sustain, but nothing too special. In dragon mode this becomes an AOE buff, just imagine your whole team running around with the equivalent of a frost hammer passive, fun times. First ability I max out, for the buff and the self heal.

Underhanded Tactics

a standard leap, and your only means of escape. The disarm seems almost like an after thought, but it is a nice to have in some situations. The main use for this ability is to get the hell out of dodge (can be used to jump in and peel off of an ally, but a situation where this will work out well for you or your ally are rare). While in dragon mode we add a kickass stun the mix making it pretty useful, but unless you are confident things are not going to turn sour before your CD is up, you might want to save it. Also when in dragon mode, this leap might as well be full on flight, it just is an insane amount of distance.

dragonic corruption
dragonic corruption
This ult is more of a buffer to your other abilities than anything special in and of its own right. It gives you a pretty nice AOE auto attack but outside of that, by itself its meh. The real use for this is a quasi-sanctuary on a much more reasonable CD. Watch out for your transformation back to dwarf mode though, you are a sitting duck for that one.

Watcher's Gift
This starter allows for you to harass the other guys and buff your buddy/self constantly. And with your passive your gold intake will not suffer at all from you completely ignoring minions.

Reinforced Shoes
I build these for the CCR, with all of the harassing your are going to be doing there will be plenty of spiteful CC's thrown your way. These boots will help with that. Alternately you could build Shoes of the Magi for some extra damage and penetration, it is purely a preference on my part to build reinforced shoes (not to mention they are 150 gold cheaper). If you are going to be adhering to my build, I would avoid the Shoes of Focus. This is because half of the CDR from that item will end up being wasted by the time you are at full build.

Breastplate of Valor
an almost must have for anyone even thinking about taking a couple hits. This item gives you a good chunk of physical protections and more than half of your CDR and all for a fantastic price.

Mantle of Discord
A very pricey, but effective protections item. Just a ton of protections, plus a "get out jail free card" as its passive it is a beautiful second protections item. Most supports cannot afford to be rocking a Mantle of Discord let alone getting it as their second item after boots. But you are Fafnir and you should have more money at any point in the game than any support could hope to have, thanks to your passive. Might as well take advantage of your built in wealth and jump straight to the cadillac of protection items.

Gem of Isolation
This item ties in perfectly with Coerce and adds some extra CC to a kit that is a little lacking in useful CC. Add in the fact that it give a decent amount of health and mana, as well as some magical power. What isn't there to love.

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What reactionary items to get and when.

Void Stone
a little magical power, a little magical protections, and some righteous magical protections shred. This is your item to get if you have a magic heavy team. It usually shines brightest in team fights so take that into account when pondering this item.

Soul Reaver
a nice to have little surprise item. It proc's from your Coerce buff, and can stack with someone else's soul reaver. So it can add some serious "SURPRISE **********ER" to any gank attempt.

Pestilence
a staple magic defense item that doubles as a hefty anti-heal item. Shines during team fights, and can shut down a healer. Usually replaces Void Stone.

Divine Ruin
another anti-heal, but with offensive stats. and guess what it works with Coerce! The extra power and lifesteal from this item can add some scare potential to this character. Usually replaces Soul Reaver. Can replace Gem of Isolation in a pinch.

Celestial Legion Helm
A different form of Void Stone. where as void stone reduces enemy magical armor, celestial legion helm gives you a very reliable anti crit (as well as a little more protections and power). This item will get picked up and replace Void Stone when the other team has completely ignored any form of magic protection and/or at least one opponent is building crit.

Spear of the Magus
more protection shred with a splash or magical power. pick this up in the odd situation where magical protections are present, but you decided not to get Void Stone (because of excess crits, healing, or you just don't like void stone). It works really well in dragon mode, because of all the tick effects, and works with Coerce; but is not as all encompassing as Void Stone. You can pick up both this item and void stone if the enemy team is really stacking magical protection and you have a very heavy magic team, but this would be an odd occurrence in my experience. Usually replaces Soul Reaver

more to be added when i damn well feel like it.