What are the conditions here.
#1 - Creature must die
(b) - You must cast this
#2 - You must cast a creature on the new slot/move an existing creature to the slot
#3 - Opponent must cast a field/mass effect spell.
That odds of that happening are terrible. Meaning the utility of this card is near zero.
Not as terrible as you might think.
1) This card needs to be in play first
2) Opponent kills one of your creatures.
3) A new creature gets played or moved to that slot.
For (3) the condition is actually not hard to do.
Unless a second creature with EARLIER priority dies, your next creature played will automatically go there.
Ex:
-You have creatures in slots 1 through 6. Your opponent kills your creature in slot 4.
-In order for your next creature to NOT go to slot 4, your opponent would have to kill your creature in slot 1,2, or 3.
In other words, most of the time, this card will protect the next creature you play after one of your creatures dies.