it is in fact "Custom Hero Arena Unlimited 2.1 and 3.0" but uhhh, there are no secret pickable heroes. There is nothing you can do to gain an unfair advantage over another player without having more skill at the game, and the knowledge of a good item build.
By this I mean, no cheats, no hacks, nothing "hidden" other than the "hidden" recipes, that are... everywhere. They are posted 4 times on this forum "officially" and about 17 times from questions like that. On the other forum, they are posted 2-3 times. Every time you play a game, they are all there.
You CAN get hidden heroes if you type -random while picking, play all random mode, or run out of time while picking, and get a random hero. You have about a 10% chance of getting one of the "special" heroes. they are not insanely strong, they are maybe... 5-10% better than the average heroes. This comes at the price of a random build though.