Skull

From HGWiki

Drop	Set-Avernus
Slot	Item

This is required to store your progress in the Hells and the Abyss, whilst is usually obtained from the Pillar of Skulls at the end of Avernus the first time you visit this area any characters above level 55 that use Reincarnation will receive one for free, which then means your first trip through the Hells does not need to begin with Avernus.