After gaining an audience with King Daniel and reaching Edge of the World, you warp to the Darklands to save Milder.
Explore the Darklands
From the transporter machine in the Darklands, go north, then east. Kill the monsters, then go south, then southwest to get the defense potion. Then go south and east.
In the northwest corner of the Darklands, there is a treasure chest that contains 500 gold.
There are many other powerups to be found around the Darklands. Just keep exploring.
If you reach a Save Machine and a set of stairs, these lead to the final boss, so don't go down the stairs unless you are ready.
When you go down the stairs, go east and check on the body. Refuse to leave, and the boss will appear. He is pretty easy, honestly. Just hit him with your strongest weapons and your strongest spells. He will sometimes stop and flash red, then cast Explosion. Otherwise he might cast Harm and Greater Harm. As you damage him, he starts to move more quickly.
See the King
After you defeat the boss, go back to Goodheart Castle and talk to a guard to ask to see the king. Talk to the king to get your reward. Congratulations!