Cult Protector:That's enough charades, priestess. We'll finish the job.
Cult Protector:Diona's done with you, misthios! Now, die!
Even the three of them together proved no match for the Eagle Bearer. Kassandra slew them all and confronted the priestess, who was cowering on the upper roof.
Kassandra:You! What's going on?
Priestess:I-I was just doing as I was told. Diona told me to take you to the garden.
She rose to her feet, but cowered under Kassandra's glare.
Priestess:Please don't kill me.
(If players asked "Why did they attack me?")
Kassandra:Why did they attack me? I freed these men myself!
Priestess:Diona played you like she played the High Priestess. The captain you killed was a leader for her sister's reinforcements. A-and the poison was to be used against you if you proved too dangerous.
Kassandra:I'll show that snake exactly how dangerous I am.
(If players asked "Why is Diona doing this?")
Kassandra:Why is Diona doing this to her own people?
Priestess:Diona hates her sister. She's been second-best all her life.
Kassandra:So she joined the Cult to take power?
Priestess:I never expected her to take it this far.
(Leave – "I'm done here.")
Kassandra:I'm going to kill this traitor.
Priestess:She's on her way to kill the High Priestess at the altar north of the temple. You'll have to hurry.
Kassandra:The altar? Good. Aphrodite will have a sacrifice tonight.
Kassandra ran out of the villa and up the road to the temple and along the way saw many injured civilians. The temple's doors themselves were destroyed, laying wide open and charred.
Kassandra:Diona! Where is she?
Priestess:She has the High Priestess. By the Altar of Aphrodite... Follow the road ahead... And hurry—
Kassandra continued past the temple and dispatched two more Cult guards en route the altar. She ran onto the altar grounds and past burning and arrow-riddled bodies. At the altar, she discovered Diona struggling with Eritha and saw they looked and dressed exactly alike.
Kassandra:You're identical twins?! Don't move or I'll cut you both down!
Unknown:Thank the Fates! They were going to kill me and blame it on you! Strike Diona down!
Unknown: She's Diona! Her jealousy and bitterness have driven her to work with the Cult. My own sister!
Unknown:This is madness! She's Diona! And she's conspired with the Cult to usurp my position as High Priestess!
Unknown:Don't be deceived, misthios! They've set this final trap to fool you!
Kassandra approached the two, comparing them.
Kassandra:I can't believe you're twins.
Unknown (left):She's always been jealous of my success, mercenary. Living in my shadow has finally broken her. Kill her, now!
Unknown (right):Lived in my shadow?! I loved you, Diona. I never wanted to be your superior.
Unknown (right): Misthios, you must kill her before she lets the Cult take over Kythera! It pains me, but it must be done.
Unknown (left):You wouldn't feel a thing, you monster—I'm not sure you ever have.
(If players asked "What was in the note from the handmaiden?")
Kassandra:The High Priestess will know what was in the note from her handmaiden.
Unknown (right):"Do not trust the Fates"!
Unknown (left):How could you possibly know that?
Unknown (right):Her end must have been so tragic she renounced the Fates. The poor girl, it was all my fault. I sent her to spy on you, and you had her killed when she threatened to expose you!
Unknown (left):Don't listen to her lies! I am the High Priestess!
(If players asked "What if I kill you both?")
Kassandra:I could just kill you both. Maybe I should.
Unknown (left):Kill us both?! You truly think they care which one of us lives or dies?
Unknown (right):She speaks true! By the Fates, with both of us dead, the Cult will simply send their own High Priestess to fill the void!
Unknown (left):You would know what the Cult would do, wouldn't you, snake?!
(If players asked "What if I walk away?")
Kassandra:What if I let you settle this yourself?
Unknown (right):Walk away and you'll have more blood on your hands! Diona will surely slay me!
Unknown (left):Snake! She knows I am the one who will perish. If you do nothing, the fall of Kythera will be all your fault.
(If players chose "Enough of this nonsense.")
Unknown (left):Slay Diona where she stands!
Unknown (right):Do the right thing, misthios! Slay Diona!
Kassandra pointed at the twin on the right.
Kassandra:You! I see right through you, Diona. It's over.
Unknown (right):By the Fates, I'll kill you both!
Kassandra successfully saw through Diona's trick and singled her out for execution. Diona fought back, but Kassandra slew her. Afterwards, she approached Eritha within the shrine.
Kassandra:Your Cult leader has been sniffed out, High Priestess.
Eritha:My own sister.
Eritha:I hated her. But, I loved her too.
Kassandra:I know how that feels.
Kassandra:Her true nature was clear.
Eritha:I loved her despite all she's done over the years. That love blinded me.
Eritha:You've secured Kythera for me, misthios. Now, about the Cult of Kosmos.
Kassandra:Good thing I killed the right priestess.
Eritha:Indeed. As High Priestess it is my duty to call out to the wayward and return them to Aphrodite's bosom. And if you've ever been to Pephka and met its inhabitants, you'll see why divine intervention is their only salvation.
[(If no quests on Pephka have been completed?)]
Kassandra:I've never been.
Eritha:Lucky you. One of my priestesses was sent there and came back... rattled. She spoke of a man—Melite.
Kassandra:What about him?
Eritha:The people living in Pephka are terrified of him. He runs some shady operations, but what exactly, I can't be sure. Maybe this will be of help.
She handed her a note.
Kassandra:Thank you, High Priestess.
Eritha:Now, if you'll excuse me, it is time to rebuild. Chaire.
Kassandra:You! You haven't fooled me. It's over, Diona!
Unknown (left):You idiot. Aphrodite aid me!
The twins scattered, and Kassandra slew the left one. On her corpse, she found a note.
Kassandra:"Don't trust the Fates?" This is the note I gave the High Priestess -
As realization dawned, she turned. Behind her stood Diona together with guards of the Cult.
Diona:By the Fates! What have you done, my beautiful stranger?
Diona:You slayed Kythera's dear High Priestess? They're going to be very cross with you.
Kassandra drew out her spear again.
Kassandra:And now I'm going to kill her sister.
Diona:I won't go down as easy. I bite, remember? I always bite!
Kassandra dealt with the guards, and killed Diona.
Kassandra killed Diona, the identical twin of Eritha and member of the Worshippers of the Bloodline branch of the Cult of Kosmos. She also received a lead on the location of another Cultist in that branch, Melite.
No matter what, Eritha is always the twin on the left, and Diona is always the twin on the right. Her verbal tic references "the Fates" throughout the quest line, hence the handmaiden's clue from The Handmaiden's Story. Ikaros can also distinguish the true priestess and the Cultist with his eagle-eyed vision.
Diona always knows the contents of the handmaiden's note, even if players chose to lie to her during Place in the Kosmos/In a Rush, which does not result in the protagonist divulging what it said. This is either an oversight, or Diona had some means of figuring out the message that is never explained.