Is a messenger warned swarm the only way for it to appear?
I've checked valora every month for several years in hideyoshi's story and it has yet to appear
Plenty of steelix and lairon though....
Yes, a messenger will inform you on the onset of the new month that a *swarm pokemon* appeared in *x area*. Swarms are pretty random and based on luck. Heck, i just got a Beldum Swarm in llusio a while ago and I didn't even need it.