Icestone

The Icestone is a dragonstone that allows a Manakete to transform into a Ice Dragon.

Fire Emblem: Monshō no Nazo
The only place where a Icestone can be found is in a particular Secret Shop. Only Tiki can use the Icestone.

Fire Emblem: Rekka no Ken
The Icestone doesn't appear physically as an item, but Nils and Ninian seal their draconic energies inside Icestones when they enter Elibe.