Depending on the pokemon, it will randomly pick a type and strength of the move. It is a very unique attack.
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In Slateport City (after you get the TM from the guy on that route), in the market place, a merchant will sell you the TM Secret Power and the TM Hidden Power.
Ancient power is not a TM in this. Hidden power however is given by the professor after 100 pokemon caught
You can buy it from a man in Slateport Market. He also sells Hidden Power.
the TM 10 in emerald is hidden power. te attack power varies among different Pokemon. accuracy:100 pp:15
there is no TM for ancient power.