In Visions of N’Zoth, the Legendary cloak Ashjra’kamas awaits you, and it brings with it a very valuable aspect: Corruption Resistance. Increasing the cloak’s level allows you to endure a greater amount of Corruption from your other gear, in addition to providing significant statistical increases and other benefits. Those upgrades to the cloak will come with time and effort in Horrific Visions, and upgrading the cloak beyond level 5 will require retrieving powerful items from the deeper (and more challenging) portions of Horrific Visions.
After your cloak has reached level 5, you can upgrade it to level 6 and beyond (all the way to level 15) as quests unlock to provide this, with a new step becoming available each week after the first. The quests to further upgrade the cloak will be sequentially available to you once they’re unlocked. If you’re still working on the quest from two weeks ago, as soon as you complete it, you’ll be able to immediately start on the quest from last week, and then the current week.
If you’re not caught up with the maximum available level of the cloak, whether that’s due to missing a few Horrific Visions at some point, or switching characters, or joining Visions of N’Zoth much later than most players, you’ll be able to complete all of the prior weeks’ quests in sequence, until you’re fully caught up.