Because that title wasn't yet entendre enough.
Alec Meer at RPS enthuses over the Endurance DLC being released with and for the PC version of Rise of the Tomb Raider (coming now or very soon and argh it's impossible to talk about this without adding more entendre, isn't it...?)
https://www.rockpapershotgun.com/2016/01/29/rise-of-the-tomb-raider-endurance-dlc-review/
It doesn't modify the main campaign, and indeed it kind of requires the main campaign to be played and finished in order to unlock its full potential. Instead, it's a loose sequel/remake of the main campaign where Lara Croft is just out in the area sandboxing around on permadeath mode, trying to keep from starving and freezing while she raids the various tombs and eventually calls a rescue chopper with a signal fire (instead of anything reasonable). Basically because she's a thrill junkie who likes to Batman herself with survival challenges.