As a Killer, your goal in Dead by Daylight is to stop the generators from being done by sacrificing the Survivors to the Entity, but you can also just Mori Survivors instead.
Mori's, full name being Momento Mori, were not added to the game until patch 1.5.3 - the reason for this being that the base game allowed Killers to choose whether they wanted to sacrifice Survivors via Hook or just kill Survivors using their signature Kill move that's unique to each killer.
What is a Mori in Dead by Daylight?
A Momento Mori, meaning Remember Death/Remember Mortality, is an add-on that Killers have. There are three types of Mori's that Killers can have equipped. These are:
Cypress Momento Mori - Gives Killers the ability to kill the last Survivor in the trial by their own hand
Ivory Momento Mori - Gives Killers the ability to kill one Survivor in the trial by their own hand
Ebony Momento Mori - Gives Killers the ability to kill ALL Survivors in the Trial by their own hand.
How to Mori Survivors in Dead by Daylight
First players will need to equip the Mori. To equip a Mori, Killers will need to go into their Bloodweb and purchase Mori's through the use of Bloodpoints, then they'll need to select the Mori of their choosing in their loadout. Once done, Killers can start the game.
To activate any Mori however, Killers will need to hook a Survivor twice. Survivors are usually sacrificed to the Entity on their third hook, but Killers are able to skip that stage by using a Mori. This prevents other Survivors from trying to save their teammate via flashlights or through Perks.
It should be noted that some Killers have two Mori's, and there are two Killers (The Executioner/Pyramid Head and The Shape/Michael Myers) that can kill you without the use of a Mori.