Kbd heads is a rare drop from the King Black Dragon. It can be combined with a slayer helmet or slayer helmet (i) to create a black slayer helmet or black slayer helmet (i), respectively. Doing so requires purchasing the ability King black bonnet for 1,000 slayer reward points.
It can also be mounted in a Skill Hall in a player-owned house. Before it can be mounted, it must be stuffed by the Taxidermist in Canifis, where she will charge 50,000 coins and return stuffed kbd heads to the player. They can then mount the head using 2 mahogany planks and 2 gold leaves. When players take it to the Taxidermist, she will exclaim, "Three?! This must be a King Black Dragon! I'll have to get out my heavy duty tools -- this skin's as tough as iron!"
The heads can be sacrificed to the Dark Altar north of Arceuus, granting 2,500 Prayer experience. This does not require or grant any Arceuus favour. They can also be split into three ensouled dragon heads by using a knife on them, which will give more total Prayer experience should the player cast Reanimate Dragon on each of the resulting heads.
For an exhaustive list of all known sources for this item, see here