The Sin Eater is an Amarr torture device. It is a sarcophagus with two metal arms ending in long needles attached. As with the Holy Star, the device is notable for its use by Vitor Dranera, who favors it when dealing with spies and other undesirables in his labs.
When activated, the needles enter the body of the person trapped inside the sarcophagus and slowly push further in, "purifying" the person inside in some manner by "eating their sins".
The exact internal mechanism, which produces a foul smell, has not been confirmed but is presumably very painful. It is not suffocation, since the device comes with an air tube. As the needles supposedly "eat" the victim's sin, it seems likely they suck out the person's bodily fluids.