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Precisely, if you build a bone pillar in the first week, the bonus will start to take effect in the SECOND week.
If you build a bone pillar in the second week, the bonus will take effect from the THIRD week.
And that, I think you'll agree, is a different matter. In the second week, you need to try your best to spend all the dark energy.
And, by the way, if you build a structure that increases the skeleton growth rate, you might not have enough sulfur for the liches (at least at the veteran level).