TomeNET

(1.3b) How to encounter the final boss: Morgoth, Lord of Darkness               
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-Morgoth is found only at dungeon level 100 or deeper.
-He won't show up until you defeated Sauron, the Sorceror, who is found at the
 bottom of Mount Doom (which is dungeon level 99).
-Morgoth does not neccessarily spawn on each fitting dungeon level,
 sometimes you have to search for him, like for all other monsters/uniques.
-If a player who hasn't defeated Sauron yet or who has already beaten Morgoth
 enters a dungeon level where Morgoth is currently on, Morgoth will instantly
 teleport to a different dungeon level.
-If there's already another player on that level who has beaten Sauron but
 hasn't acquired the title of a King or Queen yet, the 'intruder' player will
 be teleported to town instead and Morgoth will remain on the level.
-If more than one player is on his floor, he will be a bit thougher than he
 appears to a solo player, depending on the number of players facing him.
 In details, for each additional player besides the first player joining his
 level, Morgoth (assuming his defined base HP is 30000) gains +20000 hit points
 (ie +2/3 of his base HP) and +6 speed. So if a team of three challenges him,
 he will have 30000 + 2 * 20000 = 70000 HP and 30 + 2 * 6 = 42 speed.
 Everyone who is at least level 40 and on the same dungeon floor and in the
 same party as the player who lands the killing blow will get credit for
 winning the game and receive a royal title accordingly.
-For unworldy (no-ghost) parties there are extra rewards for beating Morgoth as
 a team since they cannot profit from the rekinging potential that others get
 from this. So unwordlies, if they beat Morgoth in a team, will get..
 -An extra randart drop of 'combat' treasure class.
 -Extra artifact creation scrolls for every player more than two (so trio+).
-There are no obstacles that can hold Morgoth back for long.
-All vaults on Morgoth's floor will be no-teleport vaults.
-If you die on Morgoth's floor your character is deleted even if everlasting!
 You will get a floor feeling to warn you of this whenever you enter a floor
 where Morgoth resides on, see (4.8).

(Also see (1.3a) for more information about Morgoth.)

There are rumours of even more powerful beings than the Lord Of Darkness.
Kings and Queens who seek new challenges shall try to enter the Cloud Planes
in the skies far above the world surface, where icy winds freeze you to death,
and the Nether Realm, the deepest abyss that serves as barrier of the mundane
world, a dungeon with dangers beyond imagination.
Only those who destroyed Morgoth may enter these two dungeons. The Nether Realm
can be highly rewarding in terms of experience but also very deadly. Terrible
foes are said to dwell there which aren't seen in the mundane world.
See (4.5c) for more information about the Cloud Planes and about the extra XP
bonus characters gain in the Nether Realm.