Befoul is a star within the Constellation The Shadow in Elder Scrolls Online.
With each point invested, the increase in effectiveness of healing reduction abilities diminishes. That is to say, for the first 10 CP invested, you would gain a 10.5% increase in Effectiveness. However, the next 10 CP spent (making a total of 20 CP spent), will earn you only 9.36% more Effectiveness. This is 1.14% less than the increase gained in the first 10 CP.
As such, spending just 40 CP into Befoul will already gain you a 35.23% increase in the Effectiveness of healing reduction abilities. This means you will have gained 60% of the Maximum increase possible (which is 55.0%) by spending just 40 out of the total of 100 CP. Therefore, beyond this point, the increase in Effectiveness received per 1 CP used is very small and not as worthwhile.