Where does the cost negation end I mean if i had eight blue creatures out I could summon him for free? On the plus side GREAT FLAVOR. All in all I'd give this card a firm B. If you want my opinion I'd reword it so it say Fathom Czar cost one less Blue mana for each blue creature you control you can can only cut this cost of up to four mana. And I'd make this card legendary
No offence commandant but Khalni Hydra doesn't fit as a blue card first of all because blue is an enemy colour of green and red the two colours known to have hydra's, second trample is pretty much never in blue and third blue seldom resorts to plain mana cost reduction
Same idea as Planar Chaos. Blue in the old days encompassed any form of trickery (Prodigal Sorcerer, etc.), where mana cost reduction would fit. Old blue fatties related a lot to control of islands, and tweaking this slightly, without referring to devotion, counting blue creatures could conceivably be possible. Trample, if we ignore its current associations, could be flavoured as the extra heads going around the defender, again a form of trickery.
Mostly, though, I wanted to make a simpler card, and I've always been partial to finding new roles for colours. Blue very rarely plays well with aggro and creature swarming, which was why I didn't rebalance it. Admittedly, it was a lazy design choice on my part, as Khalni Hydra was close to what I originally had.
If I can submit another card: