When the Doctor and Twilight first met. Twilight sensed that he was holding a lot of negative emotions and convinced him to open up. She hugged him and while the Doctor enjoyed the moment, he then copped a field on Twilight's rump, resulting in twilight calling him a"letch"(formal term for pervert). The Doctor later described Twilight's feeling as "Like having a marshmallow in front of a hairdryer and not having it melt" hence the nickname Marshmallow was born.
the basic Gist(so far): The Doctor arrived in Equestria,met Derpy and she became his companion(Lovestruck Derpy is a prequel to Discord Whooves following their adventures). On their final adventure,they traveled to the past when Discord still ruled Equestria, the Doctor saw a yet unknown sight called "the Truth" which discorded him, turning him selfish and cowardly. Derpy didn't like the Discorded Doctor and they broke up. The Doctor hurt Derpy, which made Derpy's friend:Warden and daughter: Dinky hate him. The Doctor later met Twilight and began a relationship with her. Twilight was attacked by the Master, and attempted suicide but was saved by the Doctor, strengthening their relationship. Recently Jack Harkness crashed into Equestria. He stayed with the Doctor for a time but during a Dalek attack, he saw the Doctor run away and even use Jack as a human shield. Jack allied with the Master to change the Doctor back to normal but, for different reasons(The Master because the discorded Doctor does not care enough to try to stop him, or save the lives of others and Jack for the same reason. Twilight revealed to the Doctor that her friends have either gone insane or have died. Pinkie killed Dash,(cupcakes style) was executed and the others drifted apart. The Doctor learns that the "grim dark" universe. A universe that's basically a hellish nightmare world filled with murderous versions of various ponies is "bleeding" into Equestria warping reality. He is currently investigating this with Twilight's help.
The blog is pretty PG, with some adult themes like fairly tamed or offscreen gore(Twilight is stabbed, a pony loses an arm etc) and some sexuality(Discord whooves tends to sleep around with various ponies and even Twilight herself). It has "companion blogs" that follow the Master and Twilight and those can get really disturbing due to the Master's violent actions and the "grim dark" world that Twilight lives in where almost everyone is either dead, insane or both. If you are attacked by a grim dark pony they will kill you, rape you, eat you, and wear your skin and if your very very lucky….they will do it in that order.There is also an archive section so that should help story wise
Ask the Time Master I think is what its called. It really depends on your tolerance for gore. Me I don't really mind that stuff, there's not much that disturbs me so I just assume on what would disturb other people.