Trying to read the emperor's mind isnt gonna cause any major problems, but its kinda fun to try. When you cast detect thoughts, he'll definitely notice and probably give you some snarky comments, since mind flayers arent exactly fans of mental intrusion. Most players report him being pretty chill about it though, just making sarcastic remarks about how hes already telling you everything anyway.

Something interesting to note - while he'll resist your attempt to peek inside his head, the whole interaction can lead to some unique dialogue moments where he calls you out on not trusting him. Its worth trying just to see his reaction, especially since theres no real downside to giving it a shot. The spell might not work as intended, but watching a powerful illithid get mildly annoyed at your attempt to read his thoughts is pretty entertaining.

No need to worry about messing up your relationship with him or anything serious - he seems to take it more as an amusing display of mortal audacity than an actual offense. Just dont expect to actually get any extra information out of his well-protected mind.