Does the spell go to your hand or is it played instantly upon sacrificing the target creature without you having to pay it's quanta cost?
I would advise against a 10% fail rate. Players tend to marginalise a number like that and take the effect for granted. In World of Warcraft the rogue class has/had an ability called Cloak of Shadows, which blocked all spells from landing on the rogue for 3 seconds also had a 10% fail rate. What happened there was that players would sometimes see the very first spell go through the CoS upon activation or see multiple spells go through it in rapid succession and rage very very very hard. You see, at 10% fail rate what you do is you take the effect for granted in order to build a tactic around the spell - similar to the human condition of taking for granted that one will not die in the next 5 seconds from an earthquake, being hit by a bus, aneurism etc.
As far as the fail rate being a tactical element for the player to think about, I don't see it. There are only 2 choices here - you either use the cauldron or not, but players will always use it. I would consider introducing a situational requirement for failure(25% for every spell the owner played this turn), softening the failure effect(the player doesn't recieve a spell, but still recieves something small, like a part of the activation cost is returned) or scrapping the failure rate altogether.
