It is not random; the skeleton summing doll is a reward for bringing the tooth back to cure the poisoned imp in the pit level of act 1. <span class='standouttext'>Spoiler : </span><span class='spoiler'>The monster was past the 2 doors with 2 levers off the southern corridor, which you need to split your characters up to get past. Blowing out the single lit torch beside a locked door past there opens it, allowing you to kill the spider and get the tooth.</span>
A stronger demonic summoning doll is in the spider forest of act 2.