Hexcrest

The Hexcrest is a unique Slayer item dropped by Jungle strykewyrms. In order to equip it, a player must have a Magic level of 20, a Defence level of 10, and a combat level of 40. The Hexcrest is also dropped by other players in PvP Worlds when the killer is wearing or carrying over 500k GP in total value (Source needed). It boosts the player's Magic strength by 15% against their slayer task monsters. It can be combined with the Focus sight and Slayer helmet to form a Full slayer helmet. With the Staff of light, a 7 level boost from an Extreme magic potion or Overload potion, and a Hexcrest or Full slayer helmet worn against an assigned slayer monster, the theoretical (if all of those things stack, which is unsure of at the moment) damage percentage boost would be 51%, potentially adding half of the total damage of already powerful spells like Fire Surge or Ice Barrage, resulting in Magic damage of approximately 42 or 45, respectively, on a slayer task, however, damage in the 90s is possible against Ice strykewyrms using Fire Surge. On the day this item was released, people were paying an upwards of 10 million coins to buy them. This was also true for the Focus sight.

Trivia

 * The Hexcrest appears higher on a player's head when seen in-game than it does in the chat window.