As title says, I killed the wraith, got the "the mighty fell" and saw the mushroom start to sprout. It only sprouted 3 of the small ones in the outer ring and never spawned the actual collectable Agari mushroom.
#Wraith at Sidae's Stake didn't create the mushroom on kill
spice sigil
not quite sure if or how this can be replicated
I was using the Maestro bow, with the mote for slow on arrow hail. Just before killing, I had done 2 arrow hails which slowed him down by a lot, then landed a headshot. Boss died from the shot + all the arrows exploding.
It recently spawned the spiderlings before it was killed. Was using Bromius, but I didn't use any pact abilities to damage the boss.