Not much has been confirmed in terms of it's function although it is suggested to be one of the Phoenix Stones. It seems that it is related to the Chimæra as Bernie Kosar became excited when he was given it and his form began to flicker.
In Chicago, Bernie Kosar is found, on the roof of Nine's Penthouse, with the antler in his mouth, howling loudly. Once he stops he tells Four and Nine through "images and feelings" that he was calling the other Chimæra although he is unsure if they heard.
It is unknown if the Antler was destroyed, following Eight's death. If it was outside of his Chest it may still be in tact.