Regigigas lives in a temple in the mountains of Gracidea. It had been asleep for centuries, for so long that moss began to grow on its body.
However, when Zero entered the Reverse World and began shattering pillars of ice within it, the glacier in front of Regigigas' temple suffered catastrophic explosions and began moving. This disturbed the great titan's slumber, rousing it into action.
When Dawn's Buneary and Swinub failed to stop the glacier with the help of a large group of wild Pokémon, including Bidoof, Bibarel, Snover and others, Regigigas appeared in the command of a herd of dozens of Mamoswine and together, they began pushing on the glacier to hold it back. Though at first they managed to still its movement and move it back slightly, in the end they too failed. It was only when Giratina healed the broken ice pillars in the Reverse World that the glacier stopped.
Regigigas returned to its temple at the end of the movie, where it is seen using Hyper Beam on Team Rocket during the credits.
Usually, Regigigas awakening requires the presence of Regirock, Regice and Registeel. Regigigas' awakening without them present may hint that there are other ways to bring it to life; however, this may also be simply from universal disturbance.
This may be a reference to the Regigigas given away as a movie promotion, which on its own can trigger the awakening of the legendary golems in Pokémon Platinum, a reversal of the usual process.
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For more information on this Pokémon's species, see Regigigas.
