Demonic skull

The demonic skull can be purchased from the Mage of Zamorak in the Wilderness for 550,000 coins. While wearing it, Runecrafting experience from the Abyss will increase from 2.5 times the normal amount to 3.5 times the normal amount. An icon is shown above players' heads when wearing the skull, and if you are killed with it in your possession, 500,000 coins will drop in its place.

However, as of 3 August 2015, wearing the skull strips away the usual Wilderness level rules - Anyone wearing it can be attacked by - and retaliate against - Any player, regardless of combat level. This makes using the Skull extremely dangerous for low-level players. The Vengeance spell, along with Retribution and Wrath prayers do not work whilst the demonic skull is equipped.

Note: The demonic skull gives no bonus whatsoever during double exp weekend.

Trivia

 * There was a bug where if you exit the Abyss into an altar then hurry back into the abyss and exit into an altar again in less than 60 seconds, the bonus experience does not refresh and you'll only receive the base experience if it expires before you craft the runes. This meant that it could be advantageous to wait until you see the message saying: "you no longer are eligible for extra runecrafting xp" before exiting the Abyss. This was fixed on the 7th of September, 2015.
 * The skull resembles Zamorak's head and K'ril Tsutsaroth's helmet.