Manifesto on Unshakable Autocracy is a 
Russian decision.
It shows up in the decision tab if Russia is an absolute monarchy and have researched Nationalism and Imperialism. It is then possible to take when they have researched Revolution & Counterrevolution. It gives 5 prestige at the expense of 3 infamy. Additionally it decreases both militancy and consciousness for all POPs in Russia by 0.5.
All in all 5 prestige is not a lot, but it might be worth to take the decision, if Russia has a lot of infamy to spare. Usually however, the infamy is best spent elsewhere.