aside from not remembering its title which doesn't help in finding it
in this story, sweetie gains an ability, granted to her by death himself. what I remember is sweetie gets precognitive abilities. in one chapter she saves the cake twins after having a nightmare of sugar cube-corner burning down.
death has tasked her with retrieving an artifact that opens the gates of Tartarus in return death with return the souls (resurrect) her two BFF's and sister as apple bloom died in a steam-powered chariot accident (caused by her)and scootaloo died in a wonder bolt training accident...
at the funeral, he gives a speech and passes out death comments on her speech while showing her friend souls
this is the most I can remember
one
it has Death Person(Equine?)ified and as the antagonist
sweetie sees the future through dreams
death has tasked sweetie in retrieving an artifact that /key to the gates of Tartarus