Sandwraiths look somewhat like floating rags with four hands. They remain hidden underground (often around a lone Bone Pile), until you or one of your pets approach them. They can drop the Spectral Matter monster charm.
In the Hall of Prometheus you will encounter a Sandwraith boss named Sandwraith Lord - Nightmare of the Desert, guarding the Portal to Babylon. He has some additional skills like the sandstorm spell and the ability to summon regular Sandwraiths.