Ending S - "Happy ending"

In order to get the Mercenary's S Ending, you must play through hardmode and obtain the King's Crown hidden in Ma'habre.

First, you must interact with Le'garde's corpse in layer 7, and then Isayah will spawn in the entrance to the dungeon. You must buy both maps and find the two buried chests in the Ancient City (See Treasure maps for more specific instructions). After doing so, Isayah will fight you and after you kill him, he will drop treasure map #3. This map leads you to the area right before you fight the Guardian Statue.

You will then need the Kings Passage Key, which is obtained by solving the statue puzzle in the Tomb of the Gods. Once obtained, go to the giant door (the one inside the hole right before the Tower of Endless), go all the way to the bottom and the crown will be buried next to the treasure chest that is right outside and directly to the left.

After that, just leave the dungeon and enjoy your Happy Ending.