Each time you defeat Iseria Queen in the Seraphic Gate, you receive a chest and 1,500,000 exp for your exp orb. Once you defeat her 10 times and receive the Angel Slayer, all successive playthroughs will result in anther Angel Slayer and the orb exp.

Angel Slayer: Beat the Seraphic Gate 10+ times.
Book of Riddles 1-8: Beat the Seraphic Gate 9 times (one book each time).
Tri-Emblem: Beat the Seraphic Gate once.