A "challenge mode" at the end of reaper dungeons.

Spawning at a low rate (~0.2%/skull up to 2% on r10), Mortality appears and offers you a challenge. Speaking to Mortality and accepting their challenge opens a portal to the land of lost souls where you can fight the "Avatar of Mortality" a huge reaper that will be a challenge to even the best reaper groups.

Their attacks can stack the epic vulnerability debuff, they have the despair aura, and can spawn vengeance circles all the while they gradually gain a melee power buff. At 75% HP they spawn yellow name reapers (Carnage, Famine, Plague and Fear). At 50% HP, they spawn orange name reapers (Vengeance and Despair) and at 25% HP they spawn Doom reapers.

Defeating the Avatar of Mortality could offer reaper fragment drops on higher skulls, as well as unique cosmetic drops that can be purchased with tokens granted by defeating this encounter, maybe some new reusable wing colours (turned in at the 3rd reaper vendor), weapon auras, mounts, vengeance, despair and doom companion pets, etc.

Feedback would be appreciated.