What is the importance of Tarquin's tomb in the novel, The Tyrant’s Tomb?

Tarquin’s Tomb is the location that Apollo, Meg, Hazel, and Lavinia go to on their first quest to find answers for the incomplete prophecy. While this does clue them in on the location of the silent god (when one of the ghouls mentions a place called Sutro), it also shows how dire Apollo’s injury is. Not only is his illness progressing faster than it should, but he is already being influenced by Tarquin as if he were already a member of the undead. This is also the place where Meg tries to fight for Apollo, believing that defeating Tarquin will save him. This does not work, and the four are forced to leave and prepare for an undead invasion.