Sorrel draws a series of symbols on the ground with some chalk, pricking his finger with a knife every few seconds and muttering arcane-sounding words. Suddenly a ghostly, transparent eagle appears in front of him. It looks at the ice for a few seconds then disappears. A second eagle follows, and flies directly into the ice. After a few such eagles have flown into the ice, Sorrel nods and does his little spiel once more. This time, an eagle appears in front of each of you and flies into the ice. You can telepathically see through the eagles' eyes to explore the chamber beyond.
The interior surface of the cave seems to be covered with a dark blue, opaque ice that sparkles in the light reflected from Legion's halo as if it were coated with a tiny film of gemstones. A promontory of such ice juts into the middle of the lake, and seems to have been shaped into an irregular pedestal, at the top of which sits a knife made entirely of the same material, with a leather-wrapped hilt. The knife is embedded point-down in the pedestal.
To the sense of the ghostly eagles, the whole room is permeated with magic. In fact, they can detect that every few seconds a pulse of magic originates from the knife and travels down the pedestal and through the cave, carried by the substance coating the walls, ceiling, and floor.
~Spellcraft #1: the school of magic on the ice is Transmutation [Earth, Cold].
~Knowledge (Nature): Sorrel is able to identify the substance coating the cave as Blue Ice ((Frostburn pg 80)).
~Spellcraft #2, Knowledge (Arcana): It seems the knife is creating a Flash-Freeze effect ((Frostburn pg 94)) every round or so, and the coating of blue ice is transferring the effect through the whole room.
Ice Dagger by
Jay PH, on Flickr