Cold

In addition to harming the hero and monsters, cold in NetHack can shatter potions, freeze watery terrain, and harden lava.

Cold damage comes from the following sources:

• Zapped or broken wand of cold
• Blown frost horn
• Spell of cone of cold
• Cold breath of a winter wolf (cub), white dragon, or silver dragon
• Melee attack of an ice troll, ice devil, or lich (any type)
• Passive attack of a brown mold or blue jelly
• Engulfing attack of an ice vortex
• Explosion attack of a freezing sphere
• Bonus damage of the artifact Frost Brand

Effects

All potions on the floor or in your main inventory can be destroyed by cold, by freezing and shattering. Potions in containers are safe.

Cold rays and explosions may freeze water, turning it into ice. Ice formed this way is likely to be temporary, melting back to water after 50-2000 turns.. Be careful when standing on melting ice! Water on the Plane of Water will never turn to ice ("The water freezes for a moment.") In NetHack 3.4.3, ice was permanent and did not melt.

The same attacks can also cause lava to cool and solidify into an ordinary stone floor. Hardened lava will not thaw out again.

Passive cold attacks

The cold-based passive attacks of brown molds and blue jellies are noteworthy: if they do damage to the target, they heal the mold or jelly and can even cause it to multiply!

Each attack that does damage adds half the number of points of damage to the monster's hit points. If the new hit point total exceeds the monster's current maximum, the maximum increases. If the monster gains enough hit points from this it will divide into two monsters of the same type.

Thankfully, cold resistance, which prevents damage from this attack, keeps such monsters from multiplying at your expense.

Cold protection

Damage to yourself or your inventory can be prevented or minimized by the following:

• Resistance: Intrinsic or extrinsic cold resistance prevents or reduces hit point loss from cold attacks. It does not prevent destruction of potions in your main inventory.
• Reflection: Cold rays and breath attacks can be reflected, preventing damage to yourself and your items. Nondirectional attacks (melee, passive attacks, explosions) cannot.
• Containers: Items in containers are safe from cold damage.
• Magic cancellation: High MC can protect your inventory from melee cold attacks. It does not prevent damage from passive or engulfing cold attacks, or from explosions or ray attacks.

SLASH'EM

• SLASH'EM features the new role of Ice Mage and the artifact athame named Deep Freeze, which deals cold damage.
• The following new SLASH'EM monsters have cold attacks: snow ants, shadow wolves, mist wolves, steel golems, crystal golems, shadows, and Vecna.
• When entering the Chaotic Quest, you will get the message, "You feel deathly cold." This coldness does not harm you or damage anything in your inventory.

UnNetHack

• UnNetHack adds a cold trap, found in Sheol, which deals cold damage in a manner similar to a fire trap.