Another example: Hearing the bigoted views of a small but vocal number of people in the UK around the time of the Brexit vote was a real eye opener with regards to how stupid people can be.

Most often used in the context of opening your eyes (making you finally notice) something hidden (often about someone). Example: Seeing his angry outburst at the poor waiter was a real eye opener for her (suggesting his temper was previously hidden from her).

An eye opener is just something, or anything that gets or grabs your attention. Something that comes as a surprise. „Wow, that was a real eye opener to see that lousy team win the game. I would have never expected that! “ And, to make „no mistake“ about something (and the phrase is usually „make no mistake about“) , is to just say whatever it is you’re referring to is certain to take place. „It’s really going to be a hot day tomorrow. Make no mistake about it! I know this for sure!“ Those are a couple examples. Hope they help.

Hey Patryk! First of all, an eye opener is "something that surprises you and teaches you new facts about life or people". (definition from Cambridge dictionary) The phrase "make no mistake", is simply another way of saying, "I'm really sure about this". So in this case, Sam is saying that he's pretty sure that this is going to be a surprising and new experience. I don't think you'll come across examples with the exact same wording, because Tolkein's writing style is rather archaic. But "make no mistake" is still used a lot. For example, you could say, "Make no mistake, you'll be late for work if you stay at the party all night". I hope this makes things clear for you. Also, if you're interested in improving your speaking skills, check out my profile. I would love to work with a fellow LoTR fan :) And, if you have any problems, feel free to drop a message.