Tier Quests

Tier Quests were introduced in the Stygian Abyss expansion. There are three NPC's that use this system:

Percolem The Hunter
Thepem the Apprentice
Zosilem the Alchemist

The way it works is you have to complete a certain amount of quests in a tier to gain access to the quests in the next tier. All of these quests are repeatable but you must wait a certain amount of time after completing a quest before you can get it again.