Lore
The Last Giant is the sole surviving Giant of the Cardinal Tower siege. He is the beaten, broken remains of the Giant Lord who had once brought wrack and ruin to the entire kingdom of Drangleic. The Giants were eventually defeated by an unnamed hero, but alas, victory came all too late as Drangleic had already become a pile of rubble. After being felled by an unknown warrior, the Giant Lord’s remains were dragged beneath the stronghold, where he was sealed away.
Many seasons had come and gone, and the Giant prepared for his final rest. When the Giants fall they grow into great trees. Death is not the end for them, for anything that has ever once lived remains a part of a great cycle of regeneration. But, the Giant Lord’s soul remained magnificent, testimony to his former strength. Perhaps, the Giants’ resentment for the King could not be pacified in death, only emboldened? A singular rage burns within their hearts and their wills of steel cannot allow them to forgive the misdeeds of King Vendrick. Perhaps it is this burning hatred that allowed the Giant Lord to remain for such a long time after his defeat? Perhaps it was targeted at the warrior who defeated him. When we stand before him, the Giant Lord recognizes us. His rage becomes uncontrollable and he is able to break free of his restraints. This final battle truly displays the determination of the race that once ruined Drangleic, as he will tear his own arm off if it will give him an advantage over you.
More likely, his endurance was simply due to the fact that his soul was still too powerful to become part of the cycle of regeneration, just yet.
More information
http://www.reddit.com/r/DarkSouls2/comments/25fxbr/giant_lord_last_giant_lore_discussion/
Theories
The Giant's motives.
The Last Giant is the Giant Lord
Sources
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