Do you remember when Randuins...

Slowed targets that attacked you? Pepperidge farms remembers. Anyway, it seems obvious but I didn't realize this at first till I did it as a joke, Rylais causes Rammus' defensive ball curl to slow attackers. It's also pretty nice for his ult in a team fight to deal more damage and slow all enemies, and while it doesn't grant any resist or armor, it does have a nice chunk of health that Rammus needs. Fun item to build after you finish your thornmail for giggles.