Let’s just say, for example, that destroying a minion flat-out is 3 mana (which is cheap, as Assassination exists for 5). Give it a downside like Execute or making it a Combo ability (e.g. Vilespline Slayer), and you can probably get away with a 2 mana destroy effect. Add in a Silence effect, and you’re knocking out deathrattles which probably ups that back to 4 (which, if it were a card itself, would still severely outclass Entomb and Assassinate, and Entomb was bonkers until the meta got aggressive).
 
Now, we have a 4/6 body, which is about 4.5-5 mana. By this logic, it should have about an effect worth 2-2.5 mana, and this is a lot more than that PLUS the benefit if it only being one card instead of two (as merging cards together is a plus, which usually results in an increased cost somewhere). This is reasonably a 9 mana card right here, maybe 8 mana if you were to really push the envelope, but for 7 mana this is a bit OP as written here as a Silence+Assassinate with a significant body.
 
I will agree that shuffling it back into your opponents deck is pretty fair instead of this “silence/destroy” combo here. That could reasonably work for 2 mana, thus balancing out the power level of the card. Even more, it still makes sense as Tartarus isn’t inescapable (as we proved with Tirek), so having the minion return is actually good flavor-wise.
 
I might also consider putting it in your deck as a “Tartarus Cell” with an on-draw effect akin to the Ambush cards. When you eventually draw the cell from your deck, the cell breaks open and the trapped minion flees into your opponent’s deck (and you draw a replacement, as with all other on-draw cards). This kind of fits the “I’ve kidnapped your minion” type theme of Cerberus a bit better than just shoving it in their deck, while still staying balanced.

@Vree  
Dood it‘s just Silence plus Destroy. Two of the most basic effects in the game. Add them up and you have a Priest spell, an Entomb alternative.
 
 
@pinkamenaislove  
I knda like shuffle into deck of player who cast this…Feels more like physically moving the card somewhere and holding the minion captive.
 
I liked the description details :D

@Vree  
It’s technically banishment to Tartarus. There is no “banishment” mechanic in the game, so I improvised. The reason it reads, silence, then destroy, is so it doesn’t trigger deathrattle,so you can think of it as the minion being locked away, not actually killed. I pondered a shuffle into the deck, effect, but it didn’t seem impactful enough.

@Vree  
Basic card? A card that instantly wipes any minion and ignores deathrattles is not a basic card!

Doesn’t suit the character flavor too well, but the effect should definitely be a basic card.