Xel'naga charged crystals are NPC units in StarCraft II: Legacy of the Void.
The charged crystals are constructs were left at Ulnar to guard the temple leading to the xel'naga themselves. The intent was that only when two races possessing purity of essence and purity of form worked together, could the constructs be defeated.
The xel'naga charged crystals are summoned into battle by xel'naga constructs, and sometimes appear independently. They are weak alone but appear in numbers. Let them group up and then use Artanis' Lightning Dash or Kerrigan's spawned banelings to kill them.