So you eanted to know the item, well here is the item, and its name is the Nemesis Seed (I know this looks really long, but please try to read it whole):

"3 months after the incident in which a higly frozen piece of DEOMEGAS blood crashed in the now baptized "Blooddrop island", scientist discovered new side effects of the event. Inside the crater left by the impact, a mysterious kind of trees started growing, which was never seen before. The trees resembled human bodies, contorned in all kind of shapes, totally dry like if they were dead. From the gray bark, a strange red sap oozed, like if they were bleeding, but the most notable detail was that each of them had a fruit, a single and odd looking berry, the "Nemesis Berry". After this first sight, furter research was programed, and the results left only more questions. The Tree never grew any leaves or flowers, and if it werent for the sap and its only berry, it would perfeclty pass for a dead tree. The berry on its own was as strange as the tree it came from. Its thorn covered skin tastes like ashes, and the pulp inside, tastes just like blood. And inside it has only one big white seed. But after many attempts to cultivate more "Nemesis trees" outside the island, there were nothing more than failures. it seemed that only the conditions inside the crater worked for the trees to grow...

But one day something incredible was going to start. A young trainer received a "Nemesis Seed" from his father who was working in the Blooddrop island. It was a gift for the boy who started his journey as a trainer that day. The boy took the seed, made a necklace with it and gave it to his newly received pokemon partner, a BULBASAUR. And after months of hard training, his IVYSAUR was about to evolve, still holding the "Nemesis Seed". But, isnted of evolving into the expected VENAUSAUR, it evolved into something tottaly different, tottaly new, and tottaly scary.

Tests were made on this new pokemon, named AUDRYSAUR and after a long investigations, the answer appeared. Nemesis Seed need more than just ground and water to grow, they feed from life force and dark energy, which were left in the crater from DEOMEGAS blood. Outside the island, the only way the seed can obtain this is by attaching themselves to a living being, in this case, a pokemon. But, if this is the case, a symbitic relation is created, and insted of becoming a tree, this seed becomes one with the pokemon, and a new pokemon is created. And up to now, it only seems to work with starter pokemons.

In order for your pokemon to evolve into a his dark counterpart with the help of a Nemesis Seed, This must be equiped to the starter pokemon from the moment it is received and before any first battle, because the seed must nusrish itself from all the energy possible before the evolving level is reached. If this inst done this way, the seed wont absorb enugh energy and wont make the pokemon to evolve."


And, sorry for the loooooooooooong description, but it was needed for this item, I know is crazy and is not a "stone" as many of you wanted it to be, but hell, it was the option I liketh the most XD.

Sorry for any typing and/or grammatical error I made, and hope you like it