Does this affect creatures that are intangible, such as the Immortal?
What about Graboids? Since they are already buried, a pitfall shouldn't affect them.
Also, creatures with the passive "airborne"?
Immortal, Phase Dragon and Graboid would be exempt, since they are from all other non-target CC, except flooding (possibly).
I did mean to state in the notes that I'm not sure where I stand on the airborne front. I think ~40% of creatures have airborne so this many creatures would be immune to it. Thematically it makes sense but I think it would be less useful, especially since you can't stop a creature from being airborne the moment it enters play. To refer back to flooding, can airborne creatures die in a flood? Apparently so.
I like the theme behind the idea, but it's overpowered. I'd reduce damage done, probably halve it, to 3 // 4, for instance, and make it not work on airborne creatures, since that doesn't really make sense. I'd still use the card under those conditions.
Could you explain why you think its OP? I know insta-kill cards are a general no-no but considering you easily bait this with something and a sensible opponent wouldn't be able to any more creatures until you did. I could easily use thunderbolt for 2|1 to cause 5 damage to a target creature, so is 3 cost to 6|8 to the next creature to be played really so bad?
That being said, whilst making this card, my though process kinda went "10 damage? Nah, too high. 6 and 8 is lower, much better." So I can't really comment