Floor

The unholy grounds of Ultima III's Castle Death were home to a unique and strange enemy. Within the stronghold of the hellspawn Exodus, the floor and grass itself would rise up to attack those who dared venture there.

Description
It is not clear if the attacking floor tiles and grasses of the Isle of Fire were some manner of before-unseen monster which disguised itself to look like the ground upon which the Stranger trod, or if vast swathes of the soil and stone which made up the castle environs were sentient beings in their own right, capable of manipulating themselves to bring injury and death to those whose footfalls landed upon them. Whatever the truth regarding this opponent, its seamless ability to blend in with its environment made it a difficult foe to confront, as its victims could only discern where to strike at it after having weathered one of its attacks.

The floor would again rise up to attack the Avatar in Ultima IX, deep within the Great Stygian Abyss.

Trivia

 * In the original game, the attacking floor appears only as the party draws near to Exodus. In the FM-Towns version of the game, however, a variety of terrain types can beset the player throughout Castle Death.
 * In the Apple version of Ultima III, floors can co-exist on the same spot, both in battle and on the Castle screen. This is different from the PC port, where they cannot stand on the same location.  This affects tactics on both views.  (Battling chests while standing on a chest in the Apple version puts you on a grass battlefield, where floors are much more visible.)
 * In Ultima V: Lazarus, a tombstone near the Bloody Plains has a humorous reference to the floor: "A knight, who had studied the lore, collected the cards, total four. He yelled at the snake to pass Exodus' lake but then he was killed by the floor."
 * Spoony of the The Spoony Experiment made much humorous commentary regarding the attacking floor tiles in his review of Ultima III, and would later be inspired to create a contest relating to the fiendish grasses of Castle Death's environs - asking his viewers to submit videos of their personal conflicts with this herbaceous monster.