Use Scarlet Phoenix Seal anywhere within the Killing Grounds to summon the Scarlet Phoenix. Defeat it, then speak with Delai.

Show Quest Info

Introduction

Delai could scarcely believe it when you told her you wish to capture the Scarlet Phoenix. She gave you the Scarlet Phoenix Seal, which can summon the Scarlet Phoenix anywhere the Killing Grounds. The time has finally come to face this fearsome foe.
Use the Scarlet Phoenix Seal anywhere within the Killing Grounds to summon the Scarlet Phoenix.

NPC Dialog

...you're kidding, right? You're actually going to
capture the Scarlet Phoenix!?
...
You're serious, I can see it in your eyes. You've
already done so much for us. Take this seal; it will
allow you to summon the Scarlet Phoenix when
you're ready. I wish you luck. If you manage to
succeed, the Fallen Angels will forever be in your
debt.

Quest Completion

I... I can't believe it. You've actually done it. We are
finally free of this tyranny that we've suffered for so
long. The Killing Grounds belong to the Fallen
Angels once again! I dare not ask your intentions,
but you have my gratitude. Thank you.