I think you mena cleanse tag. If so, you can find it at the Pokemon tower in Lavender town. It helps keep Pokemon away from the pokemno that is holding it.
Legendaries include: Latios and Latias, regirock, registeel, regice, articuno, moltres and zapdos plus raikou, suicune and entei.
you can't get them in pokemno diamond unless you migate them then go to pal park to catch them