Onyx dragon

Onyx dragon is the legendary dragon. Nobody knows if it even exists or how to find it or kill it, but it is rumoured to inhabit the worst parts of the Deep Wilderness in PK worlds. It is advised to run, log out, and not look back if you ever see it, though it could theoretically be killed by a team of players with maxed combat stats (although they might be more likely to all die instead.)

It would most likely require a team containing multiple rangers, mages, warriors, and other supporting players to drop food, potions, etc, in order to get one kill. A solo fight would theoretically be one in a ten thousand odds of success or worse, in which the dragon repeatedly hits low or misses for extended period of time.

It could potentially increase one random combat stat to maximum of 100 instead of 99 as a reward for killing it, or even weaken players as a result of being killed by it, causing them to lose experience points as a result of trauma. It can also destroy or degrade random items wielded against it such as a godsword, dragonfire shields or Barrows armour pieces as a result of ultimate firebreath, and players may find it necessary to switch between godsword and dragonfire shield every single attack in addition to prayer switching, to even damage it using melee.