Ice devils have a “hellfrost” attack that can damage you even when you have cold resistance (though possibly not if you have extrinsic cold resistance).

Ice devils should have a slowing attack, like they do in D&D. Most likely, this should be like a lichen’s temporary slow in EvilHack, rather than a permanent speed-loss like a shade or skeleton has. Free action could reduce or negate this.

Freezing a fountain does something. All the following have been suggested:

• Destroys it and turns it into an ice square.
• Breaks it and creates pools before turning into an ice square.
• Sets a flag on it which indicates that it’s frozen; it’ll thaw eventually but it can’t be dipped into or quaffed from until then.
• Creates an ice devil (a water demon that got frozen) with the same odds as getting a water demon.

Gehennom special room “ice cavern”, with mixed ice/regular floor, possibly with unnethack-style irregular ice walls, and containing ice devils and cold-themed items like wands of cold and rings of fire resistance.