An item that gives you more spirit damage the more damage you take?

[JisBeanGaming]

There is no currently an item that is specifically good for a tanker who wants to deal magical damage.
Basically the variation could be either a direct mirror opposite of berserker

1250 - Spirit Berserker (Vitality or Spirit)
+50 health
+8% Spirit Resist

Your Spirit Power increases as you take sustained damage.

+5 Spirit Power per stack
110 damage taken to stack
10 max stacks
10 second duration

or an alternative passive, if trying to be original which would be

Passive
Upon reaching 40% health instantly gain 25 spirit power.

If you want to make a more expensive version then

3250 - Spirit Berserker (Vitality or Spirit)
+250 health
+15% Spirit Resist

Upon reaching 40% health instantly gain 30 spirit power, 300 spirit shield and 300 bullet shield.