So my idea for the design of Tainted golem is based off of erosion. They have a pocket item I don't know what to name the pocket item so you choose the name.
Only rock, geodes, or fossil trinkets spawn just like normal golem but you can absorb rock, geodes and fossil trinkets with your pocket item but only with 50% of it's effect but with mom's box it goes up to 75% of it's effect.

I'm not 100% sure if Tainted golem should have the subway that normal golem has but you decide.

The gimmicks about this character.

1. Every time you get hurt your character erodes away a bit you can only get hurt 12 times before your character erodes away completely.

2. You can absorb rocks with your pocket item to replenish how many times you can get hurt but rocks don't spawn so often and tinted rocks replenish more.

3. All types of grimace won't attack you but instead will attack other enemies you can absorb a grimace with your pocket item to increase your damage your tears will have the effect of the grimace you absorbed same with the rocks grimaces don't spawn so often.

4. All types of Stoney's won't attack you you can also absorb a Stoney with your pocket item to replenish how many times you can get hurt they replenish more than normal rocks and tinted rocks and the same with the same with the rocks and grimaces Stoney's don't spawn so often.

5. Absorbing a spiky stone increases your contact damage but same with with the rocks, grimaces and Stoney's spiky stones don't spawn so often.

I'm not sure what the birthright should do for Tainted golem

But any developer of this mod who sees this you can add or take away gimmicks and things from this character or change it entirely if you want to.

You can suggest your own ideas for Tainted golem here if you want to do so.