Spell Shield is a star within the Constellation the Steed in Elder Scrolls Online.
This was added in Update 6 with the Champion System. It increases Spell Resistance by 105. You can put 100 Champion Points(CP) into this passive ability for a maximum of 5280 increase in Spell Resistance.
With each point invested, the increase in Spell Resistance diminishes. That is to say, for the first 10 CP invested, you would gain a 1003 increase in your Spell Resistance. However, the next 10 CP spent (making a total of 20 CP spent), will earn you only 897 more Spell Resistance. This is 106 less than the increase gained in the first 10 CP.
As such, spending just 37 CP into Spell Shield will already gain you a 3184 increase in Spell Resistance. This means you will have gained 60% of the Maximum increase possible (which is 5280) by spending just 37 out of the total of 100 CP. Therefore, beyond this point, the increase of Spell Resistance received per 1 CP used is very small and not as worthwhile.
In the case of most Magicka light armor builds, for each 1% increase in Spell Shield, the Spell Resistance increases by around 100. Since Spell Resistance reduces both Magic and Elemental damage, Spell Shield is preferred when the number of points required to increase Spell Shield by 1% is equal to the number of points required to increase both Hardy and Elemental Defender by 0.15%.
For a heavy armour builds, each 1% increase in Spell Shield corresponds to an increased in Spell Resistance of 200 or about 0.3% mitigation.