I started a new job, got through on-boarding and took my first ticket. Made my modification to the code base and the tests fail .. each time I run them .. on different tests .. randomly.

What the hell have I gotten myself into?

Sounds like you got yourself into a challenge.

A duel to the death, then.

It's often random until you figure out the real cause. In our projects our tests failed randomly, sometimes TestA, sometimes TestB, sometimes TestC. Until we realised that none of the tests clean up their context/data and all following tests fail assertions by a constant offset (equal to number of records created by previous tests)

Wish I could give you an advice here, but sadly I can only offer "keep fixing properly and best of luck" 👍

You fixed one test. Congratulations. You are now responsible for all our tests.