Iseria Queen is the final boss of a secret dungeon called the "Seraphic Gate" and is encountered on the fourth and final floor. When you reach Iseria, you see that she is an angelic being emitting a red aura with three pairs of wings. Hench the name "Serpahic" Gate, "A celestial being having three pairs of wings". Iseria's appearance can be described as a woman with long blond hair an a halo above her head, she is wearing a flowing black dress and holds a crescent moon staff in her right hand.
Before the battle starts, Iseria states that she is surprised that the person who brought down Gabriel Celesta has come to her, but is eager to take Lenneth on. And, with that, the battle will start. Keep in mind that Iseria Queen is much stronger then Gabriel Celesta, just about every attack she does will kill at least one of your party members. In order to win this battle you will need items like "Angel Curio" and skills like "Guts" and "Auto item" are a must. So keep this in mind when forming your battle plan.
To best Iseria, you will need to keep attacking and not even waste turns trying to heal your party, just quickly revive anyone who is killed. Once Iseria's life is down to about 50% she will transform, and a woman, similar to the one from Indalecio's Star Ocean: The Second Story transformation will appear. Iseria will now yells out: "Now I shall end this!" and performs "Empress Massacre". This attack will top well over 400,000 points of damage to any one of your party members. It must also be noted that every five turns from the start of the battle, to its end, Iseria will be using a spell called "Cosmic Spear". This attack will do around 100,000 damage to every member of your party. Seeing as max Divine Materialize Energy is 90,000, your party will die. That is. however, unless "Angel Curio" or "Guts" activates. Equipping the Sylphan Robe to your mage will negate this attack to around 20,000, allowing for survivability. Keep the presure up and you will earn your reward.
After the battle Iseria's final words are: "What do you desire after you reach the ultimate rank? Heh, it's none of my business. I'm sure... you will... be able to...". You now receive the Tri-Emblem accessory, the item has the logo of tri-Ace, creators of the Valkyrie Profile. If you choose to beat Iseria Queen again, you will receive a new "Books of Riddles" each time. On the 10th time, you will earn an "Angel Slayer", the best sword in the game.