The Master Drums.

[Thebestoftherest]

I haven't seen much of old who but I don't remember any of the old master talking about the Drums. So I will explain my head-canon for them, that they are an experiment done to him so he can't use his memories of his old regeneration as well, and couldn't turn against them as easily. I also believe that the Master eventually cure himself of that so that why Missy doesn't go on about it.

[CharlesPhipps]

It's such a weird plotpoint.

They introduce he's clinically insane because of Time Lord manipulation and then cure it without any change in his behavior.

[Jonathan101]

There is no such thing as "clinically insane". It's not a real diagnosis. Insanity is a legal issue as much as a medical one.

The Master didn't cure himself of it; he briefly mentions (at least I think he does) that the Time Lords themselves fixed his drum problem as well as his lightning / eating people problem, and then kicked him out of Gallifrey, following the events of "The End of Time".

I think it's left purposely ambiguous because it was intended to be a plot device for the Grand Finale (I assume he was thinking that far ahead), and / or perhaps Davies just wanted to give the Master some sort of "reason" for why he is a bad guy but didn't want to deal with all the problems and implications that brought up so purposely left it "vague", i.e. this "might be" what made him evil, who knows? In other words a poorly conceived attempt to give the character some depth that was never really followed up on.

In-universe the most likely explanation is either that it's a Stable Time Loop or the Doctor and the Master are just wrong- in reality he was always a bad guy, and the drums at worst just exacerbated it. Having drums in your head might drive you nuts, but there was no reason to interpret them as a call to endless war unless he himself was projecting that onto them, meaning he was already a bit like this.

Charitably we might say that as his own personal timeline got closer to the "End of Time" then maybe the drums started getting louder and louder again, and / or maybe he got therapy to suppress them but after becoming Yana they came back with a vengeance (got to wonder why the 10th Doctor was the first Time Lord in his entire centuries long life to notice that the drums were real).

But yeah, other than giving him a neat little theme tune, mostly just seems pointless. The Master makes far more sense if he is a sociopath for reasons other than "noise in my head drives me crazy".

[Thebestoftherest]

My head Canon is that the drums are newer than he believes or admit.

[Jonathan101]

Since Rassilon explicitly sent them back in time to when he was a kid, from The Masters' POV they really were with him all his life.

But Rassilon only did this because The Master was already The Master, so it's one of those wibbly-wobbly timey-wimey chicken-and-egg scenarios about which came first.

Most plausible explanation is that The Master was already evil though since according to Davies the Daleks had a different timeline before the 4th Doctor got involved in their origin and accidentally made things worse and put them on the path to warring with the Time Lords, and we already know that he had encounter an evil Master before he did that.

So...there are timelines where The Master has no drums, and timelines where he has always had the drums, and we've dipped in and out of both of them.

[CharlesPhipps]

Since we're all Trekkies here and love weird canon answers:

The drums may be why the Master went from a cultured Delgado Lex Luthor/Moriarty type to the Joker.

[Riprope22]

The drums may be why the Master went from a cultured Delgado Lex Luthor/Moriarty type to the Joker.

Yeah and every moment where the Master did something insane was because of the drums. That does explain the scheme in Time-Flight. (though maybe not the accent)

[CharlesPhipps]

Mind you, conquering evil Master is not that far off from the Timelord High Council as we find out.