In Doctor Who (2005) S9E12, the Doctor is hell bent on (among other things) chasing Rassilon off of Gallifrey. An exchange between Ohila, the High Priestess of the Sisterhood of Karn, tells Rassilon that
The Doctor does not blame Gallifrey for the horrors of the Time War.... He just blames you.
As far as I can recall, the only televised mention of Rassilon in the Time War is his plan to enact the Ultimate Sanction during the End of Time. While this would have been the greatest horror of all, the Doctor successfully thwarted the plan, and beyond causing his regeneration, had no real lasting effects.
Is there any canon material, or material in extended universe publications - novels, audio adventures, etc, that explain the horrors to which Ohila is referring, and why Rassilon in particular is blamed for them?