The most likely reasons are if it's sacrificed, if it's legendary and another legendary creature with the same name is controlled by the same player, or if its toughness is 0 or less
On the surface Darksteel Monolith seems a bit underwhelming, as an 8 mana card that lets you play a free colorless spell once a turn
What that means, dear readers, is that with Possibility Storm on the field I can cast any sorcery Adventure and I would then reveal cards until I reveal and cast the only true sorcery in the deck: Primal Surge
While you're here, you may find our beginner's guide to collecting Pokmon cards helpful - and, for a refresher on just how many Pokmon there are now, read our list of all Pokmon in this day and age (spoilers: it's a lot)
Open an entry tier: Take the lowest tier first to learn the reveal flow without high variance