Cannot be Normal Summoned/Set. Must first be Special Summoned (from your hand) by banishing 1 LIGHT and 1 DARK monster from your Graveyard. Once per turn: You can target 1 face-up monster on the field; banish that target. This card cannot attack the turn you activate this effect.
Thunder Dragons discard themselves to activate effects. Now with Thunder Dragondark they have an in-archetype DARK monster to use as cost for Chaos Sorcerer. They even trigger effects when they are banished, overall giving them great synergy with Chaos Sorcerer.