When you use the ability, what happens to the 'thrown' artifact?
It is no longer carried by the Assassin, and is uncontrolled in the target's location. Any unit may pick it up normally (including the Assassin, if it happens to be in that location).
Does the Assassin lose Stealth if it uses its ability?
Yes. It's an activated special ability, so Stealth is lost per the normal Stealth rules.
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