Cloak catchup is a JOKE

[Vadimier-tanaris (Vadimier)]

You can get one from each activity if you are behind. Source below, when Kaivax stated as much. lol

Malefic Cores And Catching Up With Corruption Resistance

We’re now a couple of weeks from some players reaching Rank 15 on their legendary cloaks, and we’d like to clarify how you’ll further improve your cloak in the weeks after hitting Rank 15. At Rank 15, players who wish to make use of more and greater Corrupted items will want to imbue themselves with greater Corruption Resistance. This is done at Rank 15 with a Malefic Core, which can be consumed and applied to the cloak, increasing the cloak’s max Corruption Resistance by 3. There are two ways …