Magician is a star within the Constellation The Tower in Elder Scrolls Online.
With each point invested, the reduction in the cost of Spells diminishes. That is to say, for the first 10 CP invested, you would gain a 4.0% reduction in Cost. However, the next 10 CP spent (making a total of 20 CP spent), will earn you only 2.1% more reduction. This is 1.9% less than the reduction gained in the first 10 CP.
As such, spending just 32 CP into Magician will already gain you a 8.0% reduction in the Magicka cost of Spells. This means you will have gained 50% of the Maximum reduction possible (which is 16.0%) by spending just 32 out of the total of 100 CP. Therefore, beyond this point, the reduction in Magicka Cost received per 1 CP used is very small and not as worthwhile.
The Magician star no longer exists in the game.