You may be looking for:
Xel'naga 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.
In an alternate timeline where Zeratul survived the beginning of the End War, Zeratul was able to deploy a massive xel'naga construct named the Avatar of Form, which could utilize charged crystals to assault Amon's Forces.
In Legacy of the Void, 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.
Charged Crystals have a 10% chance to cast a miniature Psionic Storm with each attack, dealing 40 damage to all enemy units in a small area over 4 seconds.