A Shattershard is a template SX* APX* Assault 1, One shot weapon. X* Any non-vehicle model hit by the shattershard must take a toughness test. If they fail this test they are removed from play with no saves of any kind allowed. It can only be carried by a Haemonculi and its use brings up several rules questions.
First off, it is a template, which means that you must hit as many models in the target unit as possible when used (pg29). You are dark eldar, so just move to get into a proper position to hit the special weapons and characters you want to make sure you get a chance to remove from the game.
Second. The shattershard does not cause wounds. Therefore there is no wound allocation. Meaning any model hit by it must take the toughness test. This is a characteristics test and is handled on page 8. A roll of a 6 is an autofail, meaning that even toughness 6+ creatures can be effected. A roll of a 1 always succeeds. This means that if a character is hit by the template, it cannot allocate this "hit" to another model, neither can heavy/special weapons etc be protected. Failure means the model is removed from play. Eternal Warrior will not protect you, this is "removed from play, not instant death"
Removed from play is not the same as "instant death". Instant death a rule on page 26, in which a model suffers an unsaved wound from an attack that has a strength double its toughness or greater is killed outright. Something like implosion missles use this rule, and use the instant death rule. Hence they are causing wounds that are considered double the toughness of the model hit, and must be allocated. (also in gw faq).
Now the issue comes down to the unit the haemonculi is firing from. What if his unit also fires other weapons. All shooting from a unit happens at the same time, so we resolve all the damage from it at the same time. This means that wounds from shooting can be allocated to models that are "removed from the game".
For example, you hit a unit of marines with a meltagun, sergeant, a librarian, and 3 standard marines. You rapid fire your splinter riles wounding 6 models. The shattershard when rolled removes from the game, the librarian, the meltagun, and one marine. With 6 wounds to allocate, every model that was in the unit must be allocated a wound. The librarians, meltagun are just lost wounds. The sergeant must make a single armour save, and the standard marines must take 3 (even though there are only two left).
Simple? I thought so. If you are curious to follow the link on Dakka that I had started earlier today, here it is. Dakka discusstion