There is a secret level between -5 and -7 which can be reached by using a Scroll of Height on level -7. It contains an Elder, the Thief, a Vendor who will give you 2000 gold, and a small room full of items.
Level -3 contains the Dragon Slayer, which doubles your attack versus dragons. This is required before you can defeat Morenth in level -50. It is only reachable using 2 Scrolls of Teleport.
Merchants in this Zone sell a Magic Mattock for 800 gold, and a Scroll of Earthquake for 2000 gold.
There is no altar in this Zone.
The final boss is on Level 0.
It is ideal to have the Fire Sword, and upgrade your attack & defence to 600+ by using the Altar on level -15. You can clean up some of the previous levels if need be to get more cash.
Have plenty of health 14,000+(?) - you can use the blue bottle/Well of Souls if you have one to really maximise your health but he's beatable without that.
There are 9 attackable body parts - however, first attack the feet, heart, then head (as in, just continue upwards). Once the head is dead, the other parts will die automatically.
It is possible to bypass part of this if you have a Magic Mattock, by moving to one of the rooms to either side of the boss' head and removing the wall. This is the ONLY way to defeat him if you do not have the Fire Sword, as he will regenerate his feet and claws.
He then separates into 9 pieces around the room and surrounds himself with enemies. If you have the Fire Sword, you should be able to one-shot everything - just watch out for the imps as they explode on death and you're liable to lose a bit of health. Once the 8 pieces are killed, the head is released and can be killed.
Enjoy the ending.
Secret Ending: If you've visited the Genie and chosen "I wish that none of this were real", you'll then appear in a new Genie room on Level 3 and need to kill him too. If you don't manage it the first time, you'll respawn at level 0, and should purchase a Well of Souls (using Soul Stones) from an altar room before going back to defeat the Genie.
Secret Ending 2: In the Genie room, select to wish the king back to life. After this, walk over to where the Genie was before he disappeared. The spot appears to be empty, but this activates the destruction of the lamp, to which the Genie will respond by saying "No, destroying the lamp will undo all wishes!", and a dialog box will appear telling you that you the kingdom has been restored. You will receive 13 Soul Stones and the game will end.