I want to be able to kill Caesar sneakily.

If I talk about his condition too much, he and the rest of the legion turns hostile.

How do I get the quest? Or do I have to raise my Legion reputation, first? (I'm at neutral now). I don't want to make the NCR hostile by doing too many more Legion quests, so if so I guess I just can't do it this playthrough.

You have to have a high reputation in the legion to get the Et Tumor, Brute? quest.

I think this or you have to progress further into the main quest for the Legion.
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Yep, you got to progress with the Legion questline.

Really if you get that far you have already ruined NCR and Mr House so you can only kill him via Et tumor Brute if you plan on doing Yes man or Caeasr's legion.
However Caesar is important to the Legion and Legate Lanius is a phycopath so only do this if you are evil legion or sneaky yes man.

Evil Sneaky Yes Man at that.

I THINK it's after you are given the WGS quest from Caesar, and before you do the Arizona Killer quest.

It's after the BOS part. BOS is always the last before Kimball.

To get the quest you'll have to trash your NCR and House rep.
It'll probabely trigger the no helping quests Don't Tread On The Bear, etc. So you probabely can't do it.

It's easyish to get to. Do the main CL questline, and you get it.
But make sure you have a lot of speech before you stick a scalpel in the bastards brain. Otherwise the other CL scum turn hostile. If you don't want to have a fight, that is.

Personally, there are far better ways to kill Caeser. My favourites involve a fatman, anabelle, and powerfists.

If you wanted to sneak kill the scum, you could mister sandman him in his sleep or sneak kill him when asleep. Outside of the quest you want. The idiot probabely sleeps sometimes.

I think a high medicine skill does it, too.

High Legion rep works too.

Those too. Speech stuck out in my mind because it's the most commonly used of the skills you can use.

Speech was one of the first skills I maxed out, so I'd be okay with that.

However, if I get the House/Yes-Man version of "Don't Tread on the Bear," where they just stop offering quests, I'd be okay with that. But I don't want to make NCR hostile with the Caesar's Legion version. So I'll just have to to try it on another playthrough, I guess.

I'm trying to do a Yes-Man playthrough by playing the other sides as much as possible. I've already killed House for Yes-Man, triggering both "warning" quests, so I can't really do anything else for the Legion.