It is a pokemon that dawns sister tells you about
You get it during swarm
You have to beat the game and then talk to Dawn's kid sister and she might tell you where you find a BELDUM. If it doesn't work the first time wait 24 hours and try again. I hope this helps!
Go to route 228 its in a swarm talk to dawns sister and it might take awhile
On rout 215 there is a cave go in there are a lot of beldum