This is the RSX Adaptive Histogram type version that was coded by Mladen for MT4 some time ago. Although we have a few versions of the RSX Adaptive on here, this one is the base indicator but it also is in Histogram form so in case anyone would like to use this version, i'll post it here.

For the Adaptive RSX Lines version with Multi-timeframe mode + Alerts, you can find that version here:
Adaptive RSX indicator with MTF & Alerts for MT4.

As explained by Mladen:
Adaptive rsx ...

This one is not using any of the adaptive methods that we usually use. It is using standard deviation and and an average of a standard deviation to calculate the coefficient that is then applied to the desired length of calculation. That way the length of calculation is constantly altered depending on the standard deviation and is also used as a "frame" for calculating period.

Here is a comparison of a "regular" 25 period rsx (lower) with an standard deviation adaptive 25 period rsx (upper) - as it is seen they are sometimes very different in values and I think that the adaptive is a bit more responsive to market conditions than the "regular" one: