Ven was summoned by "Ava", who I believe was not actually Ava. Already already picked her 5 new leaders and gave them instructions. She was now off doing her own thing, eventually finding Luxu to ask him questions.
Someone else disguised themselves as Ava, summoned Ven to that warehouse, tricked Strelitza into thinking Player was nearby and bringing her to that warehouse as well.
What happens beyond that is the mystery. From Ven's point of view, he blacks out right after seeing her and when he comes to, "Ava" is giving him union leader instructions.
From Strelitza's point of view, no one seemed to be in that room, but when she turned around, a shadow attacked her and then stole her book and she fades away.
I do think Ven's memories have been altered, like, was he knocked out? Did something possess him? No way he stood quietly in the darkness as he watched a girl get murdered unless something made him be silent to witness it.
The brain aches he's feeling are very similar to BbS whenever he starts recalling something painful that happened... TO him, not necessarily something he did. (Roxas too, followed by trashing a computer out of rage)
I'm 100% positive he must've witnessed the murder and his memories were altered. But, the question is, how exactly did he witness it. Did he get possessed, or something forced him to silence/freeze. Hm