King slime
From Dragon Quest IV on, regular slimes have the ability to merge to a King Slime. The game makes it appear like random behavior actually it is a different monster group from the beginning.
Slimes that will merge to a King Slime will not attack, only "call for help" till there are eight slimes. Then they will merge. They have more HP, and are fairly resistant against spells that affect a whole group. Thus the player will be able to kill off a few, but not all of them preventing the merger. It is useless to try, since individual slimes do not give experience in this encounter, only the King Slime does.
In Dragon Quest IV, the King Slime is only encountered in chapter four (The Sisters of Monbaraba) around Haville and Aktemto. It is the toughest monster to be encountered there, and gives 100 XP and 150 gold (standard slimes give 1 XP and 2 gold). It usually casts Fendspell early in battle, making it immune to magic attacks. It sometimes drops Mystic Acorns that raise max MP.
In subsequent games, the King Slime gained higher HP and rewards (115 XP, 210 gold) and the ability to block the wagon.
