Stonecoil Serpent enters and dies as a state-based action due to having 0 toughness, triggering Mortuary and Liliana, Dreadhorde General.
Resolve the Mortuary trigger, putting Stonecoil Serpent from your graveyard onto the top of your library.
Resolve the Liliana, Dreadhorde General trigger, causing you to draw a card, thus drawing Stonecoil Serpent from your library.
Repeat.
This clever loop turns the zero-mana casting cost of Stonecoil Serpent into a self-sustaining engine of perpetual motion. Because the Serpent enters the battlefield with no toughness when cast for free, it immediately heads to the graveyard, triggering Mortuary to seamlessly recycle it right back to the top of your library. Meanwhile, Liliana, Dreadhorde General watches the continuous stream of sacrifices and rewards each death by letting you draw a card, putting the Serpent right back into your hand to start the cycle anew.