Iced Shell is an Ice Magic Spell.
Iced Shell is a forbidden technique that uses the spirit and the physical body of the user to freeze the target into a block of ice. It is virtually unmeltable as it is not made of just typical ice. It channels the user's body into the form of ice, destroying their body in the process. As explained by Gray, the Caster is not "dead" but alive in the form of ice. Due to this, even Ultear could not use her Lost Magic, Arc of Time, to reverse it as it was a living thing and therefore could not be reversed or advanced through time. It is shown to be extremely durable as it did not melt for about 10 years, though Ur stated it can stay the same for all eternity. It also seems that because the ice itself is made up from the spirit and body of the caster, the target is gradually worn away by the ice encasing it.
The main difference between this and Ice-Make is that the hands must make an "X" sign (The right arm is above the left arm. The right palm facing down; left palm is facing up). The user can send out a clone, and have the clone use Iced shell.