I would like to note that this sudden attack from cats isn't entirely uncommon. A seemingly simple thing that causes any kind of change or stress can actually lead up to this without you ever realizing it was going to happen. I'm very very sorry for the loss of Basil and send many hugs to Basil's family.
I had a similar experience, but with my dog... I own a 25 lb Beagle. My aunt got two kittens when I lived with her. These two cats were raised for the first two years of their life with Charlie(Beagle). When I moved out, Charlie and I still visited with high frequency. One day, while I was watching Charlie and the female cat communicate in the way friendly animals do(him trying to initiate play and her playfully swatting at him with her paw), it happened out of nowhere. The male cat came flying down the hall, landed on top of my dog and attacked. He wouldn't back off. It happened so fast... It soon turned to my dog feeling he had no choice but to defend himself and he fought back, never actually biting the cat, but making threats while the cat clawed him up. My uncle tried to grab my dog to drag him away, but dog his hands torn open from the cat in the process. I proceeded to jump in. At this point, the female cat jumped in after the male cat and was able to distract him for a brief moment. Despite getting clawed, I was bound and determined not to let it get any further and end up with either animal severely injured or dead. I dragged my dog back into my uncles bloody hands, and while he dragged my pooch into the living room, I went for the cat who quickly tried to pursue my dog! I finally scruffed the cat, threw him in my aunts bedroom, slammed the door shut and assessed the damages.
The backs of my uncles hands and fingers were torn open(he likely should have gotten stitches, but never did)... My knuckle was torn open and I had various cuts along my hands and wrists. My dog had one long cut across the top of his nose, and a nick on his bottom eyelid(what a close one that was!), as well as another long scratch dragged down the side of his muzzle and cheek.
My dog was shaken, highly upset, but otherwise, no major damage to him. My uncle and I got the worst of it(thank god, as I live in Canada and it's free health care for me, but certainly not for my dog!). I cleaned up my dogs wounds and pampered him, before finally going off to tend to my own injuries. My knuckle should have been stitched. It was severely infected by the next day and I had to get antibiotics from the doctor to fight infection before it spread. My uncles hands healed nicely, and I still sport the scars.
The worst scars of all are the mental scars my dog now carries. He is TERRIFIED of cats. If they are laying down and not moving, he'll go say hi, but as soon as they're standing and walking, he goes into a panic attack, and tries to run away. Often ending up directly in my arms, shivering, shedding, panting and crying. If a cat persists(dog friendly cats in my family who we have been trying to desensitize him to), he was at the point where he would snap at them. He's now progressing, but extremely slowly and as much as I love cats, I will never again own or live with a cat.
I would sooner get soft paws than get my cats declawed if I ever did own any. However... I don't feel they're suited to me or my lifestyle anymore(rabbit, snake, hamster, dog afraid of cats, fiance doesn't like cats, etc).