He loops back around to that potential graveyard theme we were discussing earlier
Like most Magic: The Gathering cards, the value of Mountain depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (European Land Program)
We can only answer to the extent that we can remember
You can still play only one land per turn, and only during your main phase when you have priority and the stack is empty
In Limited, you've likely spent most of your resources in-hand just to get to a spot where you can cast the Horror as a 7/8 flash blocker, which means when you untap, you're unlikely to be able to go off with the Kraken
The foil enhances the luminous quality of the sky