This magical door is an archway that is set in solid ice. By stepping into the door, you immerse yourself in what feels like icy water, but you remain dry as you step through to the other side. One the Icegate is set in a wall of ice, it is always open and cannot be moved except by chiseling out the ice that the arch is set in.
The Icegate in action is an impressive thing to watch. As someone passes through the archway, the solid ice appears to develop thousands of cracks to maneuver around the occupant. With each tiny movement the occupant makes, the ice silently cracks and appears to shatter from within to allow unimpeded advancement through the solid ice wall. As the occupant moves forward, the fractured ice heals and melds itself back to its original form. Throwing an object through the doorway would display as a giant spider web fracture that spreads out from the point of impact and then immediately and slowly begins fusing back together into a solid wall. This will have only mildly slowed the object that passed through as if it had only passed through water.
The Icegate is nearly harmless when placed on a thin wall of ice. At worst, it chills whoever passes through. The real danger comes when the wall of ice is thick. 5-foot or even 10-foot thick walls are not unheard of. An Icegate must be placed on both sides of the ice wall to allow an entrance and an exit. Whoever enters the Icegate must hold their breath as their is no air when trapped inside a block of ice. The occupant must then swim through the receding ice and make it to the exit before they suffocate or go into shock from the bitter cold. Even creatures who can breath water are suffocated because there is neither air nor water inside the ice. For this reason, fire is always suffocated when passing through an Icegate. Once a creature passes through an Icegate, they are trapped in the ice until they find an Icegate to allow them to exit safely.
Worst of all, there is no floor through the Icegate. Once you enter, you begin sinking as the ice shatters and recedes to make way for your gravity-bound form. You must swim through the vision-obscuring cracks in the ice and fight for your life to make it out alive. If you fail, your body will sink down into the black depths of the ice with all those who have failed before you.
Rumors tell of a ice dungeon in the far reaches of the frozen tundra where a vast treasure was hidden in a room far below the ice that was only accessible by swimming down a great distance through an Icegate. More than a few adventures paid the ultimate price for seeking it out.
Additional Ideas (1)
I can see it as a de-weaponer as well. As you move through it, the ice only allows the form of a living being, any extra stuff could be stripped and left at the entrance. Either ripped or it seems like the ice has enough finesse to remove it without breaking it.