Scarlet's Game Room
Thunderfin is a level 10 Storm Creature that can be found at Summoning Pylons in Stormriven, Celestia. Thunderfin cheats but is actually kinda fun.
This boss is summoned from the Pylons in Stormriven - the first pylon on the right.
You are out of Sunstone, Scales, Sandstone, Fish Fin, and Cobalt. You might want to hunt this creature.