Symbiosis

Symbiosis lives in the Corruption biome, where its 1 in 1,331,201 base chance improves to 1 in 266,240. That is a Mythic that needs about 266,000 rolls at 1x luck during corruption or 26,600 at 10x. It came with Eon 1-16, the wiki lists it as obtainable, and proxiimai made it. The in-game text asks whether it hurt when it latched onto your neck.

It is a parasite story, and the wiki spends time on the comparison with Parasite: both are native to Corruption and both leave an infected arm. Here the player and one eye glow purple, the right arm is completely covered by a black substance that is spreading toward the torso, and an armband carries the radioactive symbol of Hazard. A purple floor sends circular slashes upward. The wiki also lists a crafting recipe that needs The Thing x50, and points out that Symbiosis stands for a parasitic relationship, which fits the hint in its description that it slowly takes over the body. No active ability.