User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-7594114-20130811072725/@comment-3146930-20130811134537

Thanatos is the one that guards the borders between the Underworld and the Mortal world. When he was captured, Gaea opened the Doors of Death causing monsters and souls to use it to escape instantly. When Thanatos was freed, he couldn't stop the flow of monsters and souls as he wasn't strong enough to defeat Gaea and retake the Doors of Death, but he can at least slow down how fast monsters reform and stop souls from escaping.

He is pretty much making sure it takes longer then a few seconds for monsters to reform and making sure those that die stay dead.