In the Sea Spirit's Den (not Seaside because Seaside is in Black 2 and White 2), you can get only 1 legendary bird, depending on your choice for a starter at the beginning of the game.
- If you chose Chespin, then Articuno will be available
- If you chose Fennekin, then Zapdos will be available
- If you chose Froakie, then Moltres will be available
However, these Pokemon are unobtainable here until you encounter them elsewhere 12 total times.
You can also find a Hard Stone or a Float Stone by using Hyper Voice or Rock Slide on a spiky rock during battle, or you could find a Damp Rock by using Muddy Water, Surf, or Water Spout on a thin rock during battle.
In the Chamber of Emptiness, you will find nothing other than a Spooky Plate, which can change Arceus to be Ghost-type, and the Banettite, which can Mega Evolve Banette.
In the Reflection Cave, you can find a number of Pokemon and items.
- You can find Mr. Mime, Wobbuffet, Sableye, Chingling, Roggenrola, Solosis, or Carbink by walking around.
- As a horde encounter, you can find Mime Jr, Roggenrola, or Carbink (Carbink is only found as a horde encounter with a Roggenrola).
- Woobat and Ferroseed have a chance of dropping from the ceiling.
- On 1F, you can find a Nest Ball, a Revive, a Moon Stone, or Reveal Glass (if at least one of the 3 Forces of Nature (Tornadus, Thundurus, or Landorus) is in your party).
- On B1F, you can find TM70 - Flash, an Escape Rope, a Hyper Potion, or Iron.
- On B2F, you can find an Earth Plate, TM74 - Gyro Ball, or an Alakazite.
Hope this helps!