The flask clinks across the cave floor undamaged, and the lone guard certainly becomes aware that he's not alone. He doesn't quite leave his cover, though he peers more intently in Mycah's direction.

Artanis finds himself mid-way through the twenty foot tunnel between Mycah and the ledge. All others remain below.

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Mycah wins initiative. Assuming he alerts Artanis, Artanis can move, but not move and attack.

None of this is sufficiently loud to make it back to the group below.