The Shantiri Crystal is a Shantiri artifact in Dark Messiah of Might and Magic.
Sareth is tasked to find it inside a Shantiri ruin where he has to fight some Black Guards for it then to deliver it to Stonehelm's Wizard Lord Menelag and to join his expedition to the Spider Temple Island to seek the Skull of Shadows.
However, Arantir's necromancers are also after the Skull of Shadows so they attack Stonehelm and kill Menelag to steal the Shantiri Crystal the night after Sareth's arrival who infiltrates the necromancers' lair under the city and steal the Crystal back from Arantir before resuming Leanna's expedition without Menelag.
Sareth then uses the Shantiri Crystal to power the Spider Temple and continue the expedition for the Skull of Shadows, implying that the Shantiri Crystal is actually a power source from the Ancient Empire.