At first I thought it was OP. Then I realized it took damage. Then I thought the unupped was UP. Then I realized that Light is the perfect element to duo with for this kind of card. Because that's where you can keep it alive with blessings and Guardian Angels
It's my understanding the health reduction is healable damage
But keeping it alive would likely still be too difficult. You could give it an active ability that toggles (like burrow) the passive on/off. A trio could use purify on it
It's health dropping would also make it rather susceptible to CC. Which isn't bad, except to make it work as I've been listing implies quite a bit of investment. It also would fall off in the late game when plenty of pillars are out, so to get one early you'd have to use 5-6 but you would have trouble keeping more than one alive
Maybe give it some innate regeneration? I'm unsure. Or a minor heal ability instead of my earlier mentioned toggle