Playing Devourer Below and one Agenda 2b it says to Test mind (6), and for each investigator who fails they must search the collection for a random basic madness weakness and add it to their hand.

my question is, where do we go from here? Must I play it immediately when my turn begins? Does it just stay in my hand?

Just treat it as if you've just drawn it, so resolve its Revelation instructions.

6 minutes ago, CSerpent said:

Just treat it as if you've just drawn it, so resolve its Revelation instructions.

Ok, I figured that is what should be done.