Minibosses

Minibosses are Mobs that are not quite Bosses but are significantly tougher than their regular counterparts. Unlike bosses they don't have an extra large health bar nor a leaderboard for who did the most damage.

Slayer Minibosses
Slayer minibosses have a chance to spawn after the player kills the corresponding slayer mob during a slayer quest of or higher. It is also possible to spawn another miniboss directly after killing one.

Dungeon Minibosses
Dungeon Minibosses can be found in regular brown rooms or yellow miniboss rooms. The Watcher can spawn in the Blood Room, and the Spirit Bear can spawn in Thorn's arena. Upon death, they are guaranteed to drop a Blessing and Essence, and sometimes they will drop keys, and even rarer encounters will give you their main item like a Silent Death or a Diamond Atom.

Broodmother

 * The Broodmother is a comparatively weak miniboss that spawns atop the Spider's Den every hour after its death. It is the only source of the Spider Talisman.