First up, if you haven't yet obtained the Lunar Wing from the Strange House , we recommend heading there now and collecting it. If you don't, you'll be unable to interact with Cresselia, a Legendary Pokemon.
This bridge also functions similarly to the Driftveil Drawbridge did, with it possible to catch Swanna flying overhead. As before, they drop feathers. If you move into a shadow on the bridge, you will either encounter a Swanna or find a feather . It's not possible to determine between the two.
Once you're ready to, head west across the bridge. As you move past the girl, she'll suddenly disappear. The ghost from Strange House ! Continue along the bridge. Partway across you'll stop and Cresselia will appear and fly down onto the bridge, assuming that you have the Lunar Wing .
Feel free to attempt to catch this Legendary. Remember to Save beforehand! That way, if you fail to catch it, then you can just restart and try again. If you want to save it for later, simply don't interact with it and come back when you feel that you're ready to take it on.
Legendary Pokemon Battle
Of all the Legendaries, Cresselia might be the easiest to catch. It has high defences, but pitiful offences, so it'll be a sitting duck (pun not intended) for you to catch. Like against the Lake trio, Dark-types will be useful for absorbing its Future Sight and Psycho Cut moves.
What can be problematic though is Moonlight , which heals Cresselia--moreso when it's nighttime. Luckily this move can only be used five times--three times if you have a Pokemon with Pressure. After that, it's straight sailing and more Poke Ball throwing.
Either way, continue west from Cresselia once you're done. Near the end of the bridge you'll find a man who's selling a Level 5 Magikarp for a cheap $500! Whoa! Hmmm, something seems familar, eh? Seriously though, these things are harder to get, being in Unova, as opposed to Kanto, so grab it if you desire.
When you're finally done, continue to the elevator at the end. Take it down to the gate below. You'll have opened up the entrance between Marvelous Bridge and Route 16! Cool! Head out to Route 16 and feel free to fly to wherever you want and do whatever you want.