Dark Eldar chicks make perfect Sslyth waifu, but suggesting this in front of them is the quickest way to get castrated.
Four arms are always better than two.

The Sslyth are xenos usually employed to act as loyal bodyguards for the Dark Eldar Archons. Apparently millennia ago, like the Eldar beforehand, the Sslyth fell to the temptations of unbridled excess and quickly became one of the most favored beings of the Dark Eldar. What a convenient marriage of two species of the same side of the coin. Sslyth typically look like snakes with four arms, which allows them to carry enough blades and weapons to end the fool who strikes at a Dark Eldar leader.

The reasons Sslyth make excellent bodyguards are their aforementioned loyalty and the fact that hiring a Dark Eldar as a bodyguard is a sure death sentence for the leader, as the treacherous and backstabby hijinks of the Dark Eldar means that a coup or assassination (or sometimes both!) is often well under way. They are also tough, shrugging off blows that would fell Humans and Eldar alike; they can even survive having one of their arms chopped off and are still 86% combat effective, which combines with their short-sighted and simple-minded approach (killing anyone who looks at them funny) to elevate their use and worth as competent bodyguards.

Although they seem to favor the Dark Eldar above all else, Sslyth as a species are mercenaries and are able to be hired if they are given what they want, similar to the Kroot, who have no genuine allegiance and just joined the Tau because they currently have the best bits, or sometimes the Imperium if they make a better offer; the Sslyth are pragmatic in this sense. It is still unknown whether the Sslyth have a thing for the Dark Eldar or if they simply share the same sadomasochistic shit.

While the Laer (the xenos also known to be corrupted by Slaanesh that were wiped out by the Emperor's Children and indirectly led to the fall of Fulgrim to Chaos) were also known to be reptilian in nature and depicted as snake-people, it is unknown if they and the Sslyth are related. With that in mind, Fulgrim's Daemon Prince form is also a four-armed snake man...