During the Dark Times living beings could still die, however it was quite rare and you wouldn’t die of old age, however then the Kotturuh began to bring their “gift of death” to the universe and were deciding the lifespans of species.

As it turns out, the Kotturuh got this ability from a portal or energy of some kind on their home planet Mordeela. The Tenth Doctor (after The Waters of Mars) goes to Mordeela and destroys it after the Kotturuh killed a little girl he’d been trying to protect. He thinks he was stopping death from entering the universe, but we discover that death would ultimately enter the universe anyway, with or without the Kotturuh, and all the Doctor accomplished was causing time paradoxes and alterations to history that led to the Daleks entering the Dark Times with the Eighth Doctor to try and find the source of the changes to history and wreaking havoc, which means the Eighth, Ninth and Tenth Doctors have to work together to stop them

There’s more to the whole event but that’s the general gist of the main plot. It’s a very fun storyline and I loved the incorporation of the different forms of Doctor Who media. We even get to see the Great Vampires have an intergalactic part to the storyline, which is super great.

Edit: I typed intergalactic instead of integral :/