Jekyll and Hyde is a random event in which Dr Jekyll appears and urgently asks the player for help. If talked to, he requests an identified herb of the player. If the player has a herb in his or her inventory, he will take it and give them a potion, but if the player has no herbs he will give the player a Strength potion (2) before disappearing, saying he isn't safe with it.
Before the Optional Randoms & Goblins update, if ignored, Dr Jekyll would turn into Mr Hyde (level 14-159, depending on the player's Combat level) and attack the player with Melee. If killed, he would drop a potion, similar to if he was given a herb. However, sometimes he dropped an empty vial. Killing him could sometimes be more profitable than giving him a herb, as he could possibly drop a Super restore potion (4).
The list, which potion Dr Jekyll gave for the herb:
The event is a direct reference to the mystery novel Strange Case of Dr Jekyll and Mr Hyde by Robert Louis Stevenson. It is about a doctor who created a potion, splitting himself into two distinct personalities. He existed as the kind, gentle Dr Jekyll, and the fearsome, malevolent Mr Hyde.
If a player were to receive this random event underwater, Jekyll would be wearing diving gear.