Tier Quests were introduced in the Stygian Abyss expansion. There are three NPC's that use this system:
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.
|Percolem the Hunter||Thepem the Apprentice||Zosilem the Alchemist|
|Tier 1 Requirement||None||None||Complete 5 of Thepem's Tier 2 quests.||30 minutes|
|Tier 2 Requirement||Complete 5 of Percolem's Tier 1 quests.||Complete 5 of Thepem's Tier 1 quests.||Complete 10 of Zosilem's Tier 1 quests.||2 hours|
|Tier 3 Requirement||Complete 10 of Percolem's Tier 2 quests.||-||Complete 20 of Zosilem's Tier 2 quests.||24 hours|