As the killer you need to unlock the Memento Mori offering and then use it in a match. After the match you will lose the offering. What it does is it will allow you to kill players crawling on the ground without having to pick them up and bring them to hooks.
Here are the three Mori offering types:
Cypress Memento Mori - Yellow offering where only the final survivor in the match can be killed
Ivory Memento Mori - Green offering where any one survivor can be killed, the first/second/third/last doesn't matter
Ebony Memento Mori - Pink offering you can kill any and all survivors
You need to kill 15 players over any amount of matches. Now because offerings are random, you'll want to kill as many survivors per match as possible. This will take you at least four matches assuming you're getting just about every survivor each match and you have the right offerings or killer abilities.
The Pig killer apart of the Jigsaw DLC has a reverse bear trap they can place on survivors knocked to the ground.
This is highly situational as they'll need to be then put in to a walking around state and only after a generator is activated will the traps start a countdown timer. Once the timer ends you'll get the kill. The problem is that these boxes with the Jigsaw puppet on top of them can be used by the people with the reverse traps for a chance to take off the trap.
I believe you only get four traps and you can not replenish them so if the survivor take off all your traps then you won't be able to get any progress via this method.
I think if a survivor tries to escape via an exit gate then trap will automatically go off before they can. Not sure about hatches.
Credit to
Scipio527 for pointing this out.
Can also be done with The Shape (Michael Myers) killer from The Halloween DLC if you have his ultra-rare add-on which you can read more about here:
Credit for pointing this out goes to
Mishqqa.
Shield Papa also wrote a Mori guide that involves the Hag:
Solution for Blood on your hands In Dead by Daylight