Parseltongue

From the Encyclopedia of Speculative Fiction - http://encyclopedia.wizards.pro

Parseltongue is the language spoken by snakes and the humans that can converse with them (known as Parselmouths). The ability to speak Parseltongue is considered the mark of a Dark wizard, which is partly due to the fact that Salazar Slytherin and Lord Voldemort both speak it. Other known Parselmouths include Harry Potter, Marvolo Gaunt, Morfin Gaunt, and Merope Gaunt.