What does the Crimson Mantle’s aging block do?

[The Pilgrim]

If you sold your soul for an insurance policy and someone threatened to rip up the paperwork, you'd be quite worried, I assume.

Still, my point stands. A Lich can't be destroyed unless his Phylactery also is. Meaning Redcloak needs to keep Xykon's phylactery at hand if he even wants to dispose of him. Which he certainly needs to in case the Plan is fulfilled and Xykon discovers that he has been fooled the whole time.