The only legendary pokemon I know of in diamond without getting a special event are:
To get Rotom, Heatran, Giratina and Cresselia you need the national dex.
With the national dex at night after 20:00 you can go to Eterna City and enter the forest. Talk to the gym leader and she'll talk about Old Gatuaue but you don't need to find it.
So go in the building on the second floor, go into the room with the tv (also bring a level 7 or 8 Pokemon with you) and go up to the tv and try to watch it will give the option to thump the tv, Rotom lvl 15 will appear he is the only one in the game and in platnium you can change his forms.
After getting him fly to Canvalve City and go into Sailor Eldrichs house (the one farthest down after you cross the bridge) and talk to the son and mum then Sailor Eldrich he will let you sail to full moon island. Then you can follow the road to find Cresseilia.
To get Heatran go to Stark Mountain and let Buck follow you then visit him after he takes the magma stone. Then go where Buck took the stone and you see Heatran lv 70 (even thought it has a gender you cant breed them).
For Giratina go to the new location on your map labelled Spring Path. When you get to the cave go forward the entire way and never go through another door, then you will meet Giratina.
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