Tenacity is a star within the Constellation The Lover in Elder Scrolls Online.
This was added in Update 6 with the Champion System. It increases the amount of Magicka and Stamina returned by Heavy Attacks. A total of 100 Champion Points can be put into this ability for a max bonus of 15.0%.
With each point invested, the increase in "the amount of Magicka and Stamina returned by Heavy Attacks" diminishes. That is to say, for the first 10 CP invested, you would gain a 2.9% increase in the Amount. However, the last 10 CP spent (making a total of 100 CP spent), will earn you only 0.2% more Amount returned. This is 2.7% less than the increase gained in the first 10 CP.
As such, spending just 32 CP into Tenacity will already gain you an 8.12% increase in the " amount of Magicka and Stamina returned by Heavy Attacks". This means you will have gained 55% of the Maximum increase possible (which is 15.0%) by spending just 32 out of the total of 100 CP. Therefore, beyond this point, the increase in Amount received per 1 CP used is very small and not as worthwhile.