Ramaladni's Gift is a legendary consumable in Diablo III.
It only has a single use, and is consumed in the process. Each Ramaladni's Gift is account-bound.
Ramaladni's Gift can only be applied to a weapon. If that weapon does not have a socket, it will gain one (as one more Primary affix, which can go above the normal 4 or 5 primary affixes limit). This socket cannot be enchanted or re-rolled. A weapon that already has a socket receives one more affix as well, but it is not yet fully clear what kind of affix it rolls (most likely, a Core stat), and unlike the extra socket, this affix is re-rollable (unless the weapon already has a re-rolled affix).
A weapon can only be enhanced once, and since it adds a non-socket affix for a weapon that already has a socket, it will never allow having two sockets on a single weapon. Note that Ramaladni's Gift can only be applied to a weapon obtained after patch 2.0.
The means of obtaining the Ramaladni's Gift are not yet disclosed, but most likely it will be a random world drop or a drop from Rift Guardians.