This is one of the game's three ending trophies, along with "Honouring Wishes" 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 top option, "Submit your Life". This is the "easy" ending for the game, as you do not have to fight any more bosses to earn it.
See the following video guide: