I was playing MTG: Arena and during mainphase 1 I casted Garruk's Horde and it wasn't countered. I expected it to cast Scaled Behemoth from the top of my deck like the card said it would but it did not. I figured there was 2 possible reasons it did not work. 1st the most likely reason is I used Sifter Wurmand set this combo up a head of time, however sense the card is already revealed Garruk's Horde didn't use it's effect. 2nd possible reason MTG: Arena is bugged or had a misruling. Do you guy's now if the card being reveal stopped the effect?
In your screenshot, you have 7 mana tapped (6 lands and 1 mana dork). I can't really tell, but were these used to cast the Horde (7 mana) or the Behemoth(6 mana)? Horde does not cast anything. It simply lets *you* cast creature cards from the top. You still need to pay the cost.
If you paid 6 mana to cast the Behemoth and it didn't go to the Stack and then the Battlefield, it sounds like a bug. But your screenshot does not show the steps you took to cast the Behemoth.
Garruk's Horde does not automatically cast the creature card on top of your library, you have to do it yourself. The ability is merely an extension of the permissions you have in the game, and this extension lets you cast creature spells from the top of your libary. This is also not a one time effect, but persists for as long as the Horde is on the battlefield and has this ability. The top card of your libary stays revealed and if it changes, the new card is revealed and stays revealed. If that is a creature card, you can cast it with the normal sorcery timing. In theory, you can cast dozens of creature cards one after the other (though still only with sorcery timing, since this new permission does not change timing only origin of spells), if they all happen to be one after the other on the top of your library. But, you still have to pay for those spells, you don't get them for free.
If you paid 6 mana to cast the Behemoth and it didn't go to the Stack and then the Battlefield, it sounds like a bug. But your screenshot does not show the steps you took to cast the Behemoth.
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