I am fairly certain that it is both. There is a timer as soon as the threshold is crossed, [b]or[/b] when that thrall spawns/ dies. Ive slow played it to the second shrieker and that door is always closed before can even see it, also have had the door close at 30 seconds as soon as that thrall goes boom.
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Could be a combination of things. I just figured this topic and random thrall would be an interesting topic. Lol