Countless ages ago a Celestial, whose true name is now forever lost to History, was murdered. By what it is not known but what is known is that it happened and the only evidence left of the crime is the Celestial's decapitated head which is slowly but surely riding the riptides of dying Universes towards the nothingness that lays beyond all of Creation.
It is believed that within the short span of a few decades Knowhere will be gone and all evidence of the murder itself will be lost.
Even though the Celestial is long dead, parts of it's mind still remain functional, as shown when Adam Warlock used the remainder to find the hidden Skrulls on board.
Following the events of The Inhuman/ Shi'ar Invasion of Knowhere, the long-dead Celestial has returned from the grave to deliver a message to Adam Warlock. The message was delivered from a alternate future from Starlord. All Celestials in all times are able to sense and communicate with one another. The message was that he is the only hope for the future.