How do I clear out this weird stuck buildup deep inside my toilet drain?

Throw on the elbow-length gloves and see if you can get a grip on it, but only pull toward you—do not push. What you're describing sounds like a chunk of an old plastic toilet brush head or synthetic scouring pad that snapped off for a previous tenant years ago. In older buildings, those nylon bristles get wedged sideways right at the throat of the trapway, catch debris, and get completely petrified in heavy **uric scale** and hard water minerals til they look like an alien crust. If you try to brute-force scrub it with a standard brush, you'll just keep snapping brittle pieces off like you've been doing. Instead, grab a pair of cheap long needle-nose pliers while wearing your gloves. Give it a gentle wiggle to test if the whole mass will slide backward into the bowl. If it feels completely cemented in place, stop yanking so you don't wedge it tighter or scratch the porcelain. To loosen the bond, you need a strong acid. Standard bleach and regular toilet gels won't do a thing to mineralized scale. Bail out as much standing water from the trap as you can using a small disposable cup and an old sponge until that lower recess is nearly dry. Then pour in a heavy-duty acid like **Zep Commercial Acidic Toilet Cleaner** or CLR directly over the blockage so it isn't diluted by water.

Let it sit for at least an hour or two. The acid will dissolve the calcium and uric crystals cementing those bristles to the glazed china, and the remaining mass should break loose easily with your pliers. Once you pull the core out, dump a bucket of clean water down to flush the remaining acid out of the trap.

Before you go tugging on it, have you completely ruled out tree roots? You mentioned ancient plumbing, and in older places, fine roots can punch right thru a failing wax ring or a hairline fracture in the trapway. They end up looking bizarrely like stiff bristles once they mineralize, and if anyone ever used those purple tank drop-in tablets, they absorb that dye permanently. If it actually is roots anchored underneath, yanking on them with pliers could easily blow out the wax seal or crack the china. Might want to take a closer look with an inspection mirror before putting tension on it.

Honestly, since you're renting, I'd pump the brakes before yanking on it. If you slip with metal tools and crack that brittle, ancient china, your landlord's 100% gonna pin the replacement cost on you. Just submit a maintenance ticket and make it thier headache. That's what you pay rent for anyway. If maintenance busts the trap trying to pry out whatever fossil's stuck down there, it's on their dime, not yours.

Update: Put on heavy gloves and gave it a tug like someone suggested, and you were right—a chunk of an old synthetic brush head snapped out! Huge relief. But the base of the bristles is still calcified and cemented to the porcelain down in the bend. Will soaking it in vinegar or CLR dissolve that buildup, or should I just call maintenance now?