After you reach level 50 by "normal" gaining experience, you have to collect rare stat-potions to advance further. To gain level 51, for example, you need first to shift to the astral plane, then locate the exit to avernus and finally the gates of Tiamat's palace. This is an extremely tough area with aggressive fierce dragons that make ancient wyrms look like sleeping kittens. Once you attack one, or they attack you they track so if possible you should set up walls and fight only one at a time. Tiamat herself has three heads which act completely independantly of each other. In addition to normal elemental breaths she can also breathe "blackness" which is unresistable and does a lot of damage. She has a few nice guaranteed drops, including artifact helmet which gives the "vampiric touch" ability, but also holds the key to the vault (undiggable, unkickable area). Inside this vault are some nice random items and a potion which will grant level 51 when quaffed.
-Sorry, got a bit carried away. Wrong game. Could have been fun though -
ADOM General's warning: Treasure Hunting causes slowness, frailty, terrible markmanship and is dangerous to your characters' health