tl;dr Aura which shows when, there is a shadow priest who used Surrender to Madness, around you.
Due to BFA changes to shadow priests, Surrender to Madness no longer kills a priest, but it damages him for 90% of their max hp (they can also decrease the damage with dispersion). That means healers would need to focus them when the buff expires.
This aura:
-shows when priest activates STM and shows his name,
-disappears after 10s, so it won't be doing a mess on your screen for a whole minute,
-appears again on 15s of STM remaining, when you should start to heal a priest.
It shows SPriests not in party too, so could be useful in warmode.
It's also okay for personal use as a shadow priest.