The Dink Network

From the portal, go down two screens and kill everything. The Flaming Sword will appear. Go back up one screen. You will notice that there are three ways to go up on this screen. The middle path leads back to the portal, but there are also two very thin paths on the left and right sides of the screen. Take the left one and go up two screens. You'll see a big arrow on the current screen, with a lot of monsters (the screen won't lock though). From there, keep going left (you'll need to go through several screens where you will have to kill everything to proceed) until you reach this screen :

Secret passage

When you do, go where the blue arrow on the screenshot points, then go left to find the secret passage that allows you to walk on part of the black area. Follow this secret passage until you reach the flaming sword.

Alternately, if you're just trying to get Ending 3, you can go back to the cathedral and talk to the red gate until it opens. Then just go north and hit the King once.