Mortarion, according to the Liber Chaotica penned by Richter Kless, is the warchief of all of Nurgle's Daemon Princes and the one held highest in the Plague God's esteem. Thankfully, the Daemon has never manifested in the material world.
Mortarion is believed by Kless to send his tendrils of horror and disease across dimensions, harvesting fear and despair for his master from multiple mortal realms. His heinous touch reaches through the Aethyr, contaminating all it passes. Kless describes strange and vast vessels passing through the Aethyr, who fell before the ministrations of this entity and became his servants, to spread Nurgle's glories across the mortal realms and the strange worlds that fill the night sky. The Daemon Prince himself is said by Kless to reside on a personal hell-world.
Trivia[]
Mortarion is likely identical to the Daemon Primarch of the same name from the Warhammer 40,000 setting.