Complete the Sinnoh Pokedex by seeing all 210 regional Pokemon, then talk to Professor Rowan in his lab.
Submitted by Professor Pokemaster
Rotom is in the Old Chateau. In order to get him (you don't need to beat the elite four) go to the top floor (2nd) go into the middle room. You have to do this between 8pm and 4am. look for a room with a TV in it. Thump on the TV
for him to go out. Rotom is Ghost and Electric and its lvl.20 so make sure you don't use a bird type but use a normal, rock, or physic and grass or ice to paralyze it and/or freeze it so its easier to catch also you need to lower it really down to health and use a really good ball like the dusk ball or quick ball or ultra ball or premier ball and maybe a timeball.
Remember to save before the battle also he can regenerate so if you defeat, flee, get defeated he'll come back if you defeat him you have to wait for the next day. If you flee just start from the pokemon center. If you get defeated go back to a pokemon center and start again.
Submitted by exploding banana
Entry location: Spear Pillar
There is a lot to do before these pokemon are yours:
1) Get the eighth gym badge, and get the HM Waterfall.
2) Go to Mount Coronet
3) Go up the waterfall, and into the secret cavern. (Just go up the mountain, you will notice the waterfall.)
4) Get the two orbs that belong to Dialga and Palkia, they are inside the small cavern.
5) Talk to the elder of Celestic Town.
6) Go to Spear Pillar! (Very top of Mount Coronet where you found the Distortion World.)
7) You will see a blue light. That's DIALGA! (After you beat/catch Dialga, enter the Mount Coronet, then step back out, there will now be a Pink Light which is PALKIA!)
Note: You might have to beat the Elite Four first, I'm not sure. Both Dialga and Palkia will be Level 70.
Submitted by GiraPalkiDia
Entry location: Hearthome City
In Hearthome City, go to the Pokemon Center, but don't go inside. Instead, go into the house right next to it. Talk to the blonde-haired girl inside. She'll introduce herself as Bebe, and then offer you an Eevee. Make sure you have room for it, then answer yes to her question. Then she'll give you an Eevee.
Submitted by secretsearcher