Elfborn is a star within the Constellation The Apprentice in Elder Scrolls Online.
With each point invested, the increase in the Spell Critical damage and Healing diminishes. That is to say, for the first 10 CP invested, you would gain a 4.78% increase in Damage and Healing. However, the next 10 CP spent (making a total of 20 CP spent), will earn you only 4.26% more Damage and Healing. This is 0.52% less than the increase gained in the first 10 CP.
As such, spending just 37 CP into Elfborn will already gain you a 15.1% increase in the Spell Critical damage and Healing. This means you will have gained 60% 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 and Healing received per 1 CP used is very small and not as worthwhile.