This is one of the game's three ending trophies, along with "Yharnam Sunrise" and "Childhood's Beginning". After defeating Mergo's Wet Nurse, return to Hunter's Dream. Go through the iron gate near the bottom of the area that is now open, and talk to the man in the wheelchair. You will be given a choice. To earn this trophy, choose the bottom option, "Refuse". You will then have to fight the man in the wheelchair, who serves as the final boss of the game. He moves very quickly and attacks with a long scythe and two different guns (though he usually does not use his guns until about 1/3 of his health is depleted). Just use the same strategies that you have acquired up until this point in the game: dodge his attacks and try to circle around to his side or back to get some attacks of your own in. Because he moves so fast, you should probably use the one-handed version of your weapon instead of the two-handed version.
See the following video guide: