Out With the Old
Plane of Ice
Optional Rule: Chill of the Frostfell
Anyone caught out of doors in the Plane of Ice must make a DC 15 Constitution saving throw each hour or gain 1 level of exhaustion. Creatures who are adapted to cold-weather environments or who are dressed in cold-weather gear have advantage on this saving throw, and creatures with resistance or immunity to cold automatically succeed on this saving throw. The frigid waters of the Frostfell are even worse; anyone immersed in the water for more than a number of minutes equal to its Constitution score must make a saving throw every minute thereafter instead of every hour, and most cold-weather gear provides no advantage at all.
Optional Rule: Walking On Icebergs
Slippery ice is difficult terrain; each foot moved across the ice costs 2 feet of movement. When a creature moves on slippery ice for the first time in a turn, it must make a DC 10 Dexterity (Acrobatics) check or fall prone. On a small piece of ice such as an ice floe, or any size ice chunk on choppy water, a character will have disadvantage on this check, and falling prone may result in falling into the water.
Thin ice has a weight tolerance of 3d10x10 pounds per 10 ft square area. If the total weight on an area of thin ice exceeds its tolerance, the ice in that area breaks. All creatures in that area fall through. In the case of fragile ice floes, this usually means falling into the frigid water.