It has just occured to me that relatively narrow triggers or targeting conditions have been used to justify cheaper costs for cards but that it hasn't really been used for rider effects. While we will occasionally see hoser effects like Stony Silence or Amulet of Safekeeping, the hosing is treated as the entire reward... which seems a little bit limiting. If someone is going through the trouble to stick a specific silver bullet into their sideboard, why not reward them for actually pulling it off?
Into the Mist
Instant
The next time that a nontoken creature would enter the battlefield this turn, if it wasn't cast from it's owner's hand, instead exile that creature and create two 1/1 white spirit creature tokens with flying.
Siphoned Inspiration
Instant
Counter target triggered ability.
Draw two cards.
Drain the Depths
Instant
Exile five target cards from target opponent's graveyard.
You may look at that player's hand and exile up to one card from it.
Shatterstroke
Instant
Destroy two target artifacts.
Shatterstroke deals 3 damage to any target.
Savage Interruption
Instant
Until end of turn, each player who has cast three or more spells this turn may not cast noncreature spells.
If any player has cast three or more spells this turn, create a 3/3 green beast creature token with hexproof.
The problem is that a lot of those hoser cards are costed at an appropriate cost without an insane benefit. If you want a hoser that doubles as a payoff then it needs to be reflected in the cost.
Into the Mist is reminiscent of Gather Specimens. Obviously because you are targeting a narrow subset instead of any creature you can reduce the cost but not all the way to 2. While I would want this at 3 it probably needs to be 4.
Siphoned Inspiration, Nimble Obstructionist costs 3 and only draws one card. There are differences such as not needing a target to draw and having a 3/1 flash flying as an alternative version is significant but drawing 1 card vs drawing 2 cards is fairly intense. Drawing 1 card isn't even worth a full mana you get a bonus effect with the draw. But bumping it to 2 and you lose the bonus and have to pay 2 more. Just drop this to draw one and its fine. If you insist on draw 2 it needs to cost 4.
Drain the Depths, other than being able to take lands this is a perfectly fine card. You can reasonably cast it turn 2 against the decks you're siding it in against but you shouldn't be able to take their lands.
Shatterstroke, Smash to Smithereens is a playable card adding a second artifact to it and allowing your to potentially take out a third target with the damage is excessive. Costing RRR is possible but I'm not sure.
Savage Interruption. This card is both surprisingly bad and completely out of color. Unlike your other cards which hate a board category this almost exclusively hates on storm. Also green isn't the color of silence regardless of what other limits you put on it. Honestly this kind of over powered but narrow use card already exists in green. Veil of Summer
I agree with the above mostly. Especially the green one, which I can see it trying to punish storm, just isn't great and will really only be a speedbump, at most.
I do disagree a little on the blue one, but mostly just because triggered abilities are so common in the game that this just becomes Divination with a minor bonus and is pretty easily main-boardable. Cost it at 1UU and its fine.
Into the Mist
Instant
The next time that a nontoken creature would enter the battlefield this turn, if it wasn't cast from it's owner's hand, instead exile that creature and create two 1/1 white spirit creature tokens with flying.
Siphoned Inspiration
Instant
Counter target triggered ability.
Draw two cards.
Drain the Depths
Instant
Exile five target cards from target opponent's graveyard.
You may look at that player's hand and exile up to one card from it.
Shatterstroke
Instant
Destroy two target artifacts.
Shatterstroke deals 3 damage to any target.
Savage Interruption
Instant
Until end of turn, each player who has cast three or more spells this turn may not cast noncreature spells.
If any player has cast three or more spells this turn, create a 3/3 green beast creature token with hexproof.
Into the Mist is reminiscent of Gather Specimens. Obviously because you are targeting a narrow subset instead of any creature you can reduce the cost but not all the way to 2. While I would want this at 3 it probably needs to be 4.
Siphoned Inspiration, Nimble Obstructionist costs 3 and only draws one card. There are differences such as not needing a target to draw and having a 3/1 flash flying as an alternative version is significant but drawing 1 card vs drawing 2 cards is fairly intense. Drawing 1 card isn't even worth a full mana you get a bonus effect with the draw. But bumping it to 2 and you lose the bonus and have to pay 2 more. Just drop this to draw one and its fine. If you insist on draw 2 it needs to cost 4.
Drain the Depths, other than being able to take lands this is a perfectly fine card. You can reasonably cast it turn 2 against the decks you're siding it in against but you shouldn't be able to take their lands.
Shatterstroke, Smash to Smithereens is a playable card adding a second artifact to it and allowing your to potentially take out a third target with the damage is excessive. Costing RRR is possible but I'm not sure.
Savage Interruption. This card is both surprisingly bad and completely out of color. Unlike your other cards which hate a board category this almost exclusively hates on storm. Also green isn't the color of silence regardless of what other limits you put on it. Honestly this kind of over powered but narrow use card already exists in green. Veil of Summer
I do disagree a little on the blue one, but mostly just because triggered abilities are so common in the game that this just becomes Divination with a minor bonus and is pretty easily main-boardable. Cost it at 1UU and its fine.