Equipment in the game is class-restricted, ergo ->
1. Warriors can equip all.
2. Rangers can equip light and medium-weight, but medium when they specialize, after lvl 15.
3. Mystics can equip only light-weight.
Mystics are not mages, nor wizards or warlocks.
They are more like entities, which can enhance their willpower, inner spirit, focus and concentration, as well as those of their allies. And vise versa - to diminish their foes' spirit and willpower.
They suggest, bless, buff. And they curse, banish, cast fear into the hearts of their enemies.
They don't throw out fireballs or chain lightning from their hands.
There is no magic here, which you are used to see in Harry Potter, WoW or Lineage.
Ergo -> The Staff of Thousand Trues doesn't exist. The only item that can enhance the Mystic's special abilities... is his mind and his will.