Aegis is an enormous shield that has a secondary usage besides defending. Aegis has the power to heal the user and replenish their energy.
When Sir Homkronite was still alive, he used Aegis and his golden sword and armor to defend his town. In his afterlife, his shield remained in the town shrine.
Gerosin finds Aegis in a shrine in Homkronites' home villiage, and the shield is then officially given to him by Curdam. Curdam used a charm that binded Aegis to Gerosin's spirit, so that all Gerosin needs to do is shout the name of his shield, and it will come to him, making it easier to carry around the big heavy shield.