The Beast of Legend is the second side quest received in Egypt (Act II). It is given by Seda, a desert trader who made camp at the entrance of the Wadjet Canyons (north of Hathor Basin). Seda and his companions give some hints about a scorpion monster lurking somewhere in the desert.
That monster is the Ancient Scorpos - Beast of Legend (Insectoid), which can found in a cave some distance to the north. The reward is given upon killing the monster. Returning to Seda won't give any further reward aside from congratulations.
Reward: 7500 Experience, 2 skill points