ok how about this for the Blood Ravens solo mode ability?

Choir of Angels

It has been know that on occasion Blood Ravens can hear the emperors will made manifest, the choir of the astronomican, through the warp during battle. They enter a trace like state which greatly improves their prowess.

A blood Raven can, once a game session, enter a battle trace that greatly improves his focus and combat ability, he gains a number of rerolls equal to half his rank to be spent on any roll, that must be spent by the end of the current engagement or they are lost.

gabriel angelos does this a number of times in the books