User blog comment:Imrulkays1/Reward for killing Kalameet?/@comment-3236835-20150711075117

You get ~8,000 souls (more if you can equip the Covetous Silver Serpent Ring fast enough before receiving the souls after the battle), and the Obsidian Greatsword (good luck severing his tail >:) ).

You also get the Calamity Ring, but I guess you'd much prefer a kick in nuts rather than wearing that abomination.

It's a hell of a fight, and even the most experienced players must resort to play strategically. I've never seen anyone who claims this boss is easy.