Mondain

Location Mondain's Lair
Notes Kill spell makes him stronger; casts psionic shock (damage), mind blaster (reduces all stats), magic missile (damage)
Mondain's spells don't appear to require him to fire them in the 8 main directions, which seems a bit unfair. Of course, it's pretty easy to beat him to a pulp despite that, so it's OK.
Compare Mondain to other Creatures
Guard Mondain (Bat)

Related

Lands of Lord British

Page 2
Win the Red Gem in The Castle of the Lost King and obtain a vehicle that can cross water.

Lands of Danger and Despair

Page 6
Win the White Gem from Shamino in his castle.

The Final Battle

Page 7
Use the four Gems and the Time Machine to battle Mondain, destroying his Gem of Immortality and then defeating him.

Mondain (Bat)

Location Mondain's Lair
Notes Kill spell makes him stronger; runs away from you
When Mondain is hurt enough, he turns into a Bat and starts running away. Leave him alone, and he will turn back. It can be frustrating to chase down the Bat, but it can be done. Keep up the fight and he will fall without casting another spell.

Mondain

Location Mondain's Lair
Notes Attack is greatly weakened when gem is destroyed, uses spells at range (magic missile and psyonic shock deal damage, mind blaster reduces all stats by 10%)
Mondain's normal attack is devastating, dealing hundreds of damage per hit until you destroy his gem. Even then he's no slouch, and he can cast spells if you walk away from him. He doesn't need to fire them on a line, but your ranged attacks don't need to be used on a line either, so that's no problem.

Mondain (Bat)

Location Mondain's Lair
Notes Runs away from you, returns to human form if left alone long enough
When Mondain is hurt enough, he turns into a Bat and starts running away. Leave him alone, and he will turn back. Your best bet is to chase him into a corner and pound him into submission with ranged Attacks.