The Lifestream is a magical source of power in the Afterlife, the more recent kills in this realm the more energy the Lifestream contains. Sadly for players this is a bad thing as the more energy in the lifestream the more powerful the monsters become in this realm.
This means the monsters are weakest at the start of the day but as the day progresses it gets tougher and tougher, until potentially a point where the monsters can not be defeated.
For every 2500 kills in the lifestream the monsters are 10% more powerful, this means with 10,000 kills in the lifestream the monsters are twice as powerful.