Seeker Chariots of Slaanesh

When the Daemon armies of Slaanesh need something with a little more oomph to their cavalry forces than Seekers of Slaanesh, they hitch two Steeds of Slaanesh to a chariot.

Fluff

The chariot is designed to be "appreciated" in motion as an abomination of shifting shapes due to the bizarre structure of the wheels and the polished metal of the vehicle itself casting reflections and shadows that strain the sanity of the viewer. The axles cause a screeching noise that is appreciated by Daemonettes, but is a nightmarish cacophony to everything else (so punk rock wheels). The Daemonettes aboard are comfortable with the movement and literally dance back and forth as the cart jostles across terrain, slashing at foes and enjoying the ride every inch of the way. The spiked bottom of the cart and the blades on the wheels turn the vehicle into a thresher that runs over foes.

Heralds of Slaanesh sometimes choose to use a Seeker Chariot as a mount, and will pluck victims from the masses running from the blades to...enjoy...in vile ways while the battle rages if they get bored going at several hundred miles an hour in an open vehicle.

On The Tabletop

Warhammer Fantasy

Seeker Chariots are an upgrade from Seekers of Slaanesh, and aren't special enough to really merit much discussion otherwise. More expensive in points, slightly more killy, fulfills the same role. Take it if it suits you, or skip it in favor of Fiends of Slaanesh the same as you would Seekers.

The 9th Age

Same as Fantasy, take them as an alternative to Steeds. They got even more destructive with a buff however, but as they only exist in a Unit of one they actually become the superior Hammer than a Unit of Seekers. If the Fast Cavalry rule is what you need, or if you intend to use a larger Unit or a Herald in a bunker, go with Steeds. If you need something that hits hard with some staying power, use Fiends. But if you need something fast that can bring the pain, Chariots are the way to go.