I like it! It also reminds me of Tupilaks. Do a quick Google search. If I understood it currectly, they are monster statues carved from bone that inuit shamans threw to the waters, thinking the ritual would enable them to send a horrible creature to eat their opponent.
Hrmmm, thats pretty interesting there. Might take a look into'em.

Oo, I know what he should do next! Vampiric clothing! Seriously, it's kind of hard to dodge your set of full plate.
Its been a long time since I've seen Libris Mortis, but I think there is a creature called a Raiment that is just that.

Do any of your Dying Ember PCs have kids? I think this monster works best if your PCs are attached to some weak little friend or child, and must protect their child from such predatory spirits.
Nope, but one of the main NPC's in one group is an Elan who was expiremented on as a child and thus only ages mentally, but still has the body of an 8 year old. Not standard Elan lore, but I'm the DM ;)

Also have a gnome in the other group, so both are ripe for encountering one of these (though probably most likely the Elan).

Its ambush abilities and damage output are somewhat worrisome for a CR 4 monster- it seems the kind that could do significant damage, even to a 4th-level party, with little trouble. Like, kill a PC with its sneak attacks and aquatic abilities. In a straight fight, it's not a CR 4 by any stretch, but I would strongly doubt that a Qalupalik would ever consider a straight fight while it had some water to escape into.
Aye, thats why I went with 4. Its strong in some ways, weak in others, though its natural attacks aren't all that particularly strong.