This area's pretty small and only covers a single floor. Head north-east and enter the door along the east wall. Move east and enter the next door. Turn on your Dowsing MCHN on and find the hidden Sun Stone in the centre of the room, then continue east through yet another door.
Head east once more, then grab the Ultra Ball in the north-east corner of the final room. Backtrack to the room with a door leading north and enter it. Run north to the Pokemon at the end of the hall. Feel free to fight it. It's very rare, so if you want to catch it, save first so that you can retry if you fail to.
Note that it's not easy to catch, so you need to make full use of status ailments and Poke Balls. You might also need a fair bit of patience.
Wild Pokemon Battle
Once you're done, backtrack all the way back to the Relic Passage . From there, either head back to the Pokemon World Tournament or, if you're feeling hardy, cut through Castelia Sewers and into Castelia City . Then you're free to do whatever you want; might be a good time to return to where you left off in the story.