The Vision Crystal is an artifact required for any champion to successfully complete their Ascension. the Crystal will focus the gaze of the Gods of Tyria on the champion as they face their doppelganger in the final test of Ascension and showcase their skill.
Historically, the Crystal used to be in the possession of the Elonians who mistrusted each other and thus kept its three Crystal Shards separate so that not one of their factions would have ultimate control over it. In the end they were overrun by their enemies and never made use of the shards.
The Vision Crystal is assembled in the Elona Reach mission. Players start out with one shard and need to obtain the two remaining shards from two opposite camps swarming with Forgotten and Enchanted within a limited timespan. Once players complete this mission successfully, they will find the assembled crystal at the shrine at the base of the mesa in the Skyward Reach outside of Augury Rock.
The Crystal shards can also be used to revive the Priests in that mission to complete the bonus.