u/IntroductionPale8234

I have an 18-year-old cat (neutered male), and I adopted a new cat a few weeks ago (3-year-old spayed female). My old cat has always been very friendly with other cats, and when the new cat got here he was good, very interested in her, sniffing and cautiously approaching. She was understandably nervous, a bit of hissing and yowling. But not too serious, and I felt like with time they’d both settle in. And they were…slowly but surely they could sit on the couch together, and they even both slept peacefully in my bed together just a couple of nights ago. Now just today, all of a sudden, my old boy has started attacking her, running at her with claws flying, and of course she reacts badly, screaming and fighting back, and finally retreating to the basement (her safe place). I have no idea what suddenly changed or why my old guy got so agitated all of a sudden. I don’t want to have to keep them separated but I’m worried he’s just going to keep tormenting her. Any advice is welcome, thanks!