Literally my favorite item (I would not like to fail ability checks)

does this stack?

It does stack. 

It doesn’t work with skill checks does it?

It should. Skill checks are a type of ability check. 

Keep in mind that initiative is an ability check.

so is counterspell

Is that Deep Speech?

That doesn’t look like an agate at all.

it cant stack cuz its attunement, DMG page i think 161 (dont quote me been a while) says that you cant gain benefits from attuning to multiple of the same item, like 3 rings of protection dont give +3 AC

I thought it meant the eyes are polished agate, but now that I read it again, it probably should just be a chunk of pure agate. 

Quote from GrumpyMidget >>

Is that Deep Speech?

 What?

Quote from Eravanfeighn >>

I thought it meant the eyes are polished agate, but now that I read it again, it probably should just be a chunk of pure agate. 

 Maybe only the eye is the item, and the cat skull is from the item being from some context where the item is found in a stone cat skull's left eye socket?

Attunement items dont stack with identic magic items.

how do I add this to equipment in homebrew backgrounds?

One of my favorite items from 3.5 days.

Really, my DM says it isn't.

Skill checks fall under abilities. It does It also grants a bonus to initiative modifier as that is a ability check. You are welcome. :) 

And this helps on Death Saves too!

No, death saves are saving throws, not ability checks.

Edit: I completely missed the fact that it says saving throws, it does in fact work on death saves too then