Thaumaturge is a star within the Constellation The Ritual in Elder Scrolls Online.
With each point invested, the increase in damage over time diminishes. That is to say, for the first 10 CP invested, you would gain a 5.0% increase in Poison, Disease and Magic Damage. However, the next 10 CP spent (making a total of 20 CP spent), will earn you only 3.1% more Damage. This is 1.9% less than the increase gained in the first 10 CP.
As such, spending just 37 CP into Thaumaturge will already gain you a 12.5% increase in DOT. This means you will have gained 50% of the Maximum increase possible (which is 25.0%) by spending just 37 out of the total of 100 CP. Therefore, beyond this point, the increase in Damage received per 1 CP used is very small and not as worthwhile.