The Bittercold looks like a giant snowflake with six prongs, six more purple prongs extend behind these. At it's center is a pink diamond shaped core encased in ice.
It has an eerie purple aura emanating from it's core and expels chunks of ice from it's center.
Plot and/or ending details may follow.
The Bittercold is said to have been born from the negative emotions of Pokémon. The dark aura emanating from it's center is known as The Winds of Despair and drains the energy of any Pokémon who gets to close.
The Voice of Life, Hydreigon tells the player that he brought them to the world of Pokémon because The Winds of Despair doesn't have much effect on humans. Kyurem guards it because he wants to protect the future even if that future is the world's destruction. Because of this, no human has ever gotten close enough to defeat it even though many others where brought to the world.
As soon as the player and their partner approach The Bittercold, their partner collapses due to The Winds of Despair.
As the player continues to approach even they begin to feel the effects of The Winds of Despair but the power of encouragement from the other Pokémon soon suppresses them and the first part of the battle begins.
After a few turns, the player realizes their attacks are not working and begins to lose hope and The Winds return.
With the encouragement of the other Pokémon and their partner the player manages to muster up enough strength to smash the casing around the pink core. The darkness over the tower dissipates and a small rainbow appears over the town. The other Pokémon seeing this then begin to regain hope and encouragement.
The player feels this too and the final battle begins. The player defeats The Bittercold by shattering it.
After this The Winds once again dissipate and Glacier Palace begins to fall from the sky.
Spoilers end here.