The buff goes away on long rest.
After completing the creche area, you're forced to long rest when you leave Mountain Pass.
You might be able to do the creche, leave, then come back and get the crest to obtain the buff and take it with you to somewhere else. I haven't tried personally, but theoretically should work if you for example, wanted the extra radiant damage going into the Shadow Cursed Lands where everything is weak to it.