WE DON'T HAVE ANY GRAVYYARD HERE. DEAD CREATURES ARE GONE UNTIL THE END OF MATCH!
Well, as of now. You could make the card store a variable, like "All creatures are assigned a number, when one dies on your side, change the variable to said number, ability usage makes the respective creature appear in your deck and clears the number". Of course, it sounds like something real impossible to code even now, but...
WAIT. THIS THING IS BETTER THAN A FIRE SHIELD FOR SAME 6 COST. BAD.