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Spoiler
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I am assuming that you are clearing out Bodhi and her guild in Chapter 3. Have you killed Lassal yet? He is the male vampire that taunts you. You need to go downstairs where he will ambush you with more vampires. After surviving the ambush, Lassal will retreat back upstairs to the Spike Room. Enter the Spike Room( carefully, as there are undisarmable traps) and kill him.
You should be able to stake the third coffin now.
Cheers. [img]smile.gif[/img]