I want it pls.
first you need to have caught or defeated dialga on spear pillar.
Then go to the route between Veilstone city and Pastoria city. You'll find a passage somewhere between to right if you're coming from top. Go inside and surf to a cave. Around the cave, the lake's name is lake valor. When you enter the passage, it will show lake valor so confirm this way. Inside you'll find azelf. Be prepared, it's not easy to catch.
Hope I helped!