A Fiora that stacks Tiamat has her ult, and NOTHING ELSE AT ALL she can do. She is an absolute one trick pony, nothing broken about it.

How do you figure. It applies to her basic attacks as well. 4 Tiamats and a Bloodthirster for sustain. Not to say its foolproof, my team did manage to get her when our Blitz opened with CC, like he shouldve been doing all along. But she could literally ace my team by herself. Myself included if I weren't smart enough to wait out of range until she had used her ult. Don't forget there is still the rest of her team to contend with. Her squishiness led every death I dealt her, but it wasn't making up for the fact that she took my entire team with her nearly every time. Besides, it is really just basic math. I mean, Rabadan's Deathcap doesn't stack anymore, and it only gave you 30% AP bonus for each one. An additional 50% AD for every Tiamat, and AD translates 1 to 1 for damage on EVERY HIT? How is that in any way balanced?

Yes, it does apply to her basic attack, but if a Fiora has 4 tiamats, every halfway competent team that is not ridiculously underfed (I am talking about 20k gold behind-dimensions) should be able to explode her within a timeframe of 0, 5 second if she goes into a fight (and that is a serious estimation, in no way exaggerated).

One stun and she should be dead and a stun can often be landed before she is able to unleash her ultimate and even if she does unleash it, Tiamat's range is ridicullously low, so if you are all lumped up close enough to be wiped out by it, it is amatter of your team needing to improve your positioning, not a matter of Tiamat Fiora to be broken.

Stuns did work, yes. I'm not really making the case just for Fiora, this was just an example. I actually just got back from testing the Tiamat with Garen, and while it didn't seem to make a huge difference in fights with bots that stood and fought back, as soon as they turned to flee I noticed a MASSIVE increase in the damage my autoattacks were doing. It might have been the crit chance in my build, I will try a build without just to be sure, but my autoattacks seemed to be doing more damage than Decisive Strike with a Trinity Force built whenever they were moving, leading me to believe that there may be a delay between when you hit and when the splash is applied. Thus allowing you to destroy fleeing opponents with more ease than was probably intended.

The real question I have is, does Tiamat splash affect the target you are attacking, or just other targets nearby? Because if your target gets the bonus splash damage IN ADDITION to what you are hitting them for, then you are effectively multiplying your damage output by stacking Tiamats, which would absolutely be broken. Especially considering how relatively cheap a Tiamat is.

EDIT: doesn't appear to have any effect on the target you are hitting, only targets nearby. Still, I think the splash radius really needs to be increased to make it useful, but it would just be way too overpowered if it were still allowed to stack the passive. The AD and regens are fine where they are.

You are right, the splash does not hit the target you hit for additional 50% damage, if it did, it would be core on any carry.

The passive should be unique. Tiamat now is not broken, and in my opinion it is underpowered. It is not bad on some champions like Fiora if you stack it but on most it just isn't strong enough to purchase. If they buffed it though it would be OP for these champions. In order for it to be a plausible purchase for most, but not OP on some the passive must be made unique.

Also as suggested by ModelT in another thread(Tiamat Touch Up) the item should have a particle animation for the splash damage.

This, basically word-for-word. Sorry if I have lead anyone to believe I am raging at Tiamat. To sum up my thoughts, it just isn't viable as an item unless you are abusing it by stacking the passive and coupling it with certain exploitative on-hit abilities. By that definition I think it is broken, in the sense that it is too underpowered to be useful in and of itself, without being paired with some manner of game mechanics exploit.

I remember one troll game where me and a friend were Shyvana and Fiora and each got 3 timants.

We lost but whenever my friend ulted or I q'd in dragon form the other team was hurting.