http://fairytail.wikia.com/wiki/Ice-Make:_Prison

or

http://fairytail.wikia.com/wiki/Iced_Shell

?
because the second one says:
Iced Shell (絶対凍結・アイスのドシェル Zettai Tōketsu: Aisu no Sheru): Gray also has knowledge of this forbidden, extremely dangerous and powerful Magic, and has expressed the certainty that he could use it, if he were forced to

it also kills the user...