Arach d'Kundarak

Bogwater Tavern
Arach can be seen sit in a reserved table, talking to a red-haired elf woman. If you approach, you can hear their dialogue, one phrase at a time:
 * Woman: The key to the Prophecy is in the Vault—will you not help me, my love?
 * Arach: I won't have any problems getting in—after all, I designed the vault!
 * Arach: The golem will help when the time comes...
 * Arach: If I do this, my brothers will never forgive me.
 * Woman: Yes, yes, I know your house of bankers will not approve of our liaison. But does it matter?

If you try to talk to them, however, each one will try to expel you from the proximities:
 * Arach: The lady and I are trying to have a conversation here.
 * Woman: We desire privacy—leave us be.

Although this is a foreshadowing for his betrayal on Kundarak, they never leave the tavern, even after you complete the Plane of Night.

Vault of Night

 * Arach d'Kundarak: I am Arach of House Kundarak... it is I who designed this place... and it is I who showed the Aurum how to take it.
 * Arach d'Kundarak: Why, you ask? -- My duty to Eberron superseded my loyalty to House Kundarak and my dragonmark. It is vital that Velah decipher the mystery of the Dragonshards, or this world is doomed...
 * Arach d'Kundarak: So, are you here to execute me for treason?
 * You: House Kundarak will handle your punishment later. But, you will open the Gate of Night now, or will die!
 * Arach d'Kundarak: Your threats bore me. My knight and his Hounds will put an end to your interference!

After the battle is finished:
 * Arach d'Kundarak: Now that I have seen you fight, I am even more convinced that Lord Barrow sent you to your deaths. If you have such trouble with clockwork contraptions of gears and hedge-wizardry, how can you hope to challenge a creature like Velah?
 * Arach d'Kundarak: I tell you for the last time—you do not know what you are facing! You should leave this game to those who understand the stakes!
 * You: Open the gate, Arach. I will not rest until I have finished this.
 * Arach d'Kundarak: I will open the gate—but do not expect to return this way if you fail!